Connect the audio Outputs of the playback device to the MONTAGE A/D In
Set the front panel A/D INPUT ON/OFF switch = On and set the Gain as appropriate

In a User Performance, press [EDIT] > on the upper “Common/Audio” level of the architecture touch “Audio In”
Set the A/D In “Output Select” = AsgnL&R
Press [STORE]

If you would like to make this Global for all Performances...
Tap the “FX” icon top center of the screen (or go to [UTILITY] > “Effect Switch”...)
Set the “Global A/D” = ON

Making it “Global” will allow you to freely utilize the MONTAGE without interruption of your external audio when you change Performances. If “Global” is not set On, you can determine on a Performance by Performance basis how the signal is routed.

Bad Mister, thanks for your reply. Your solution takes into account using an external device.

But, what if I wanted to playback files stored on a USB stick and route them through assignable L&R outputs?

You’re welcome. The ‘playback device’ must be an external device. Only the internal synth Parts and the external audio inputs (A/D In and Digital In) can be routed to the AsgnL&R Outputs. Streaming audio from a USB stick is simply routed to the Main L&R Output

To gain an understanding of the Audio Routing possibilities when using the MONTAGE as your Audio Interface:
Go to [UTILITY] > “Settings” > “Audio I/O” > see the Audio Signal Flow diagram for the MONTAGE routing possibilities.
All signal travels left-to-right in the signal flow diagram. The diagram will show your current conditions.

Audio returned via USB can be routed to the “Main”, “Digital In” or “Assign” (when you setup your Audio Outputs from your DAW you can select between these three Stereo options) — once you select a DAW Output option, the signal can be routed either to Main L&R Outputs (Analog Out), or to the Assign L&R (Analog Out).

There are personal and professional (for DJs, for example) audio devices that will play MP3, WAV, etc. audio files from a USB stick and output to analog outs. You could purchase one of those and use one of these devices to play file(s) on your USB stick and route those to the A/D inputs which can then be output to the assignable outputs.

Certainly you can use a computer as well to serve as a player for content on the USB stick but a more dedicated device may take up less space, have "zero" boot time, and generally may be more reliable and easier to deal with on stage.

Thanks Bad Mister and Jason!