I just figured out something about this. With Audio Monitor OFF, the faint sound goes away if I set the volume level on my Windows PC to zero. The sound is coming out of the speakers attached to the Montage, not the PC speakers.

I still have no idea why this is happening, but perhaps a better idea about where it’s happening.

I’m not sure I follow exactly what your Setup is (not at my gear so can’t look at your attachment quite yet)..., but audio does return from the DAW to MONTAGE - there is a USB VOLUME control in MONTAGE. This allows you to hear the Cubase “Click”, for example.
[UTILITY] > “Settings” > “Audio I/O” > USB VOLUME... Perhaps you are hearing audio returning through this path. There is a shortcut to “Monitor Volume” which routes/controls USB Audio to either the Main and/or Asgn Outs...

I found that the same thing happens whether the Montage speakers or the PC speakers are selected in the Sound Control Panel's "Playback" tab. Weird.

Have you had a chance to try looking at my project?

Must be something you are not mentioning that is in play... or a Windows issue.
What is the Sound Control Panel's "Playback" tab have to do with audio in Cubase?

Are you using something, perhaps, not mentioned in this thread to route audio in your WINDOWS environment?

I hear no leakage.

It’s very strange.

I turned off power to the speakers that are hooked up to my Montage via Output (Balanced). Nothing is hooked up to Assignable Output (Balanced).

In the Montage Audio I/O settings, I have USB Volume set to zero. For good measure, I have both Main and Assign also set to zero in the Monitor Volume settings. The volume of the leaking audio is affected by the USB Main dB setting. Direct Monitor is OFF.

In the Windows 10 Sound Control Panel, I have Speakers set as the Default Device. These are the speakers that are part of my Surface Pro tablet. When I hit a key on the Montage, I can see audio activity in the vertical display on the right side of the Speakers device.

In the Cubase project, I have Stereo In, Stereo Out, MIDI and Audio muted. The MIDI track has Monitor On.

The sound I hear through the PC speakers is slightly delayed. The sound is definitely being generated by the Montage. I verified this by switching the one and only Part of the Performance from a piano sound to a flute sound.

I have no idea how sound generated by the Montage is getting into the PC sound card and speakers in this situation. It seems to me like it shouldn’t be able to do that. But it’s definitely happening.

I’d be really happy to hear an explanation of what’s going on.

In the Windows 10 Sound Control Panel, I have Speakers set as the Default Device. These are the speakers that are part of my Surface Pro tablet. When I hit a key on the Montage, I can see audio activity in the vertical display on the right side of the Speakers device.

Why do you have this setting?
Aren’t you using Cubase with the YSUSB driver?
Who’s “driving”?

“Slightly delayed” leads me to believe you are not using the driver, and are using whatever the default driver that comes with the tablet... clearly when not using the YSUSB driver you are in the realm of a Microsoft issue... I’d reach out to them. The MONTAGE is just the source of signal in this case. Is it not?

In the Montage Audio I/O settings, I have USB Volume set to zero. For good measure, I have both Main and Assign also set to zero in the Monitor Volume settings. The volume of the leaking audio is affected by the USB Main dB setting. Direct Monitor is OFF.

Let’s understand each of these one at a time so you can figure out your mystery.
“USB Volume set to zero” — this is audio returning to the MONTAGE when using the MONTAGE as your audio interface. This is located between the audio returned to the MONTAGE as audio interface and the Analog Outputs. (You are not using this at all).
“Main and Assign also set to zero” — this also is audio going to the Analog Outputs of the MONTAGE. Audio that originated in the computer and is returned via the USB. (You are not using this at all).
“Volume of the leaking audio is affected by the USB Main dB setting” —this setting controls the signal level traversing the USB connection. As signal (in your setup) is getting to the computer speaker whatever driver you are using should be in charge of receiving and regulating signal. The source of the signal is the connected device (MONTAGE).
“Direct Monitor is OFF” — This disconnects the internal MONTAGE sound engine from sending analog audio directly to the Analog Outputs... this would be used when you are going to set the YSUSB driver to monitor through (post) the computer DAW. (As I read your post this also is not used at all)

You are not using the MONTAGE as your audio interface (which means you have a customized setup) what/how is Audio routed? What is the application? What are you attempting to route? Are you attempting to use the built in speakers? Why?
It comes down to “who’s driving the bus?”
You mentioned having the tablet’s speakers selected as the default device (I assume that’s an output setting) but you make no mention of how you have the input setup -which is what you want to concern yourself with (you know signal is getting to the speakers) you want to know how it got IN THERE. (Make sense?)

Weren’t you doing some customized rerouting of computer audio a few posts ago - check you haven’t overloaded something internally?

And (importantly) does any of this occur when you are properly using the YSUSB driver and connected to monitor speakers via the MONTAGE Outputs?

It's simple.
You just have incredibly sensitive hearing Michael 😉

Why do you have this setting?

It was just one of the many different configurations I tried in the process of trying to figure this out. I get exactly the same delayed sound when I set Line MONTAGE to be the Default Device in the Windows 10 Sound Control Panel.

Aren’t you using Cubase with the YSUSB driver?

I am definitely using the YSUSB driver with Cubase.

You mentioned having the tablet’s speakers selected as the default device (I assume that’s an output setting) but you make no mention of how you have the input setup -which is what you want to concern yourself with (you know signal is getting to the speakers) you want to know how it got IN THERE. (Make sense?)

Assuming you mean the Recording tab of the Sound Control Panel, Line MONTAGE is set as the Default Device in that tab as well.

And (importantly) does any of this occur when you are properly using the YSUSB driver and connected to monitor speakers via the MONTAGE Outputs?

It does. I can see now that it would have been better for me to describe that particular configuration instead of the one I described. I only described the other one because that was where I ended up when I came to the strong conclusion that I would not be capable of figuring out what was going on, and needed to ask for help.

I like to draw pictures if I do not understand something. Something with all of your audio connections, digital connections, and computer settings which are part of the audio path (or possible audio path) along with the Montage-side parameters noted which can change/redirect/mute/etc. the various audio paths.

Also, on the PC side - routing is different if you have the Yamaha Steinberg USB driver activated vs. the generic ASIO drivers "loaded". Things also change if you use something like ASIO4ALL or VB-Audio's VoiceMeeter. Using either (ASIO4ALL or VoiceMeeter) introduces many more permutations because there are more configuration possibilities meaning that each should be understood or at least the one you're using would need to be documented.

There's not quite enough information to draw all of this for your setup yet.

Also, it is important through this process to internalize which parameters on your Montage and PC apply to adjusting audio OUTPUT (from montage to PC -- or from PC to Montage depending on context) or INPUTS. If you get this wrong or do not understand then adjusting a parameter may not be doing what you think.

That was a good clue, Jason.

I just uninstalled VoiceMeeter, and the audio leak went away. I wasn’t using it anyway, so no great loss.

The things that can happen. Yikes!

Yes, it's important to follow the audio "bouncing ball" all the way from initial generation to final termination. If the PC is in the loop - then anything that can contribute has to be understood. There is a fairly narrow window of productive use for something like Voicemeeter. When problems surface, it's always best to scale back to a minimum configuration. I installed Voicemeeter as one of a couple ways to solve a user request of how to play youtube videos out Montage using a digital path (with other requirements on the user's request). Voicemeeter was installed to help with that thread - but I have since removed Voicemeeter because youtube or PC sounds making their way out to Montage in combination with anything else is not how I use the instrument.