In order for your PC to send it's audio out to MODX - there are things on both sides that can trip you up.

On MODX, be sure your MIDI mode is set to USB (and not the MIDI port). There should be a USB logo symbol on the top line of your MODX touchscreen and not something that looks like a circle with some dots on it (a USB 5-pin DIN looking symbol).

A quick check would be to go to your PC and hover over the speaker icon (or right click the icon which should bring up the volume slider) -- assuming you're using Windows. Not sure really what you have. When you hover or left-click, the driver name is shown. Right now I don't have my keyboard plugged in so the audio driver shows my TV manufacturer as the name. Maybe yours says something else like "Realtek High Definition Audio". Regardless if it doesn't say "MODX" (probably with a dash and a number) - then something is wrong.

After checking the MODX midi mode is USB - check the PC side and just make sure the MODX isn't one of the selections for sound cards. When I left-click the speaker icon (near the time in the lower-right taskbar) and it shows the driver name and volume slider - there's also an arrow that looks like "^". If you click this arrow it will show all other drivers you can choose from. Maybe your default driver is just set to something other than MODX. If this is true, you'll see MODX in the list and can select it.

Certainly, making sure your PC volume isn't muted is something to check. This is one of those "is it plugged in" suggestions - sorry if too basic.

MODX can also have settings that will alter the audio routing. A couple of things to try if sound still isn't happening (but the PC does have "MODX" as its driver):

1) Try using the Performance "Init Normal (AWM2)". This will make sure the Performance settings are not what is messing up the routing.
2) Try setting the quick setup to "Standalone". This will reset many of the system-level configuration options and put those to the factory setting without having to do a factory reset.

3) As a last resort, you could backup your MODX and then do a factory reset (initialize all). The problem is unlikely to be at this level - so I would explore the other possibilities first.

Bought a MODX8 a few weeks ago. Just learning the piano.
The sound on my pc and MODX worked fine for a couple of weeks.
Now, when I turn on the MODX, I get sound from the instrument. But, no sound from my PC.
Any suggestions?

We have to assume some things you have not told us about...

1) That when you say you get “sound from the instrument”, that the MODX is connected to a sound system separate from the computer’s sound system. Hopefully, a pair of studio monitor speakers connected directly to the Main L&R Outputs of the MODX. Yes/no?

2) That when you say “no sound from my PC” that you are referring to things that would normally play on the computer’s sound system, and were coming through the MODX are no longer coming through the MODX — this could simply be a Volume control issue. Audio coming in from the computer has a separate Volume Control referred to as “USB VOLUME” in your MODX. It has its own dedicated Knob on the left front panel (right next to the MODX Main Volume Knob).

3) some applications allow you select audio option within the program... like Cubase AI 10... when you select the “Yamaha Steinberg USB Driver” as your ASIO Driver, the MODX will take over routing of computer Audio back through the MODX. If you are not running Cubase, and this is what you mean when you say “no sound”, you can set you computer’s sound options via your Windows settings... you can elect to send them through the computer’s built-in system, or through the MODX as an external option.