I'm not sure how to stop the DAC from shutting down when Mint goes to sleep, but maybe you can avoid the problem by making Mint not go to sleep in the first place? There's an option for it in the System Settings program, in the "power management" menu (under the "Hardware" heading).

Try doing that and see if the problem keeps happening. If it keeps happening, the DAC shutting down was probably caused by something else.

https://ubuntu-mate.community/t/how-to-enable-the-s-pdif-output-to-be-permanently-active/26220

Maybe this? I think spdif is a more relevant search term, toslink didn't seem to turn up much.

Disable auto-off power management features for the port you have the device plugged into. Try the 'Tunables' menu in the powertop utility if you want to temporarily test to see if it works. Easiest thing I can think of for a beginner.

I think it might have to do with the lock screen. Try locking the screen while audio is playing and see what happens. If that is the case I think there might be a workaround that involves reverting to the old lock screen or something. Hope this helps..

See if there's a way to disable power save for your audio driver module.

I had to do this for Intel for example

#/etc/modprobe.d/audio_disable_powersave.conf
 options snd_hda_intel power_save=0

Step one is probably figuring out why the Mojo is turning off. If it's the USB power going to sleep, the answer could be as simple as running the power USB cable directly to an outlet instead of the computer (which is often better for audio quality anyways). Otherwise, I'd look into the spdif link that the other person posted.