John, pardon if it seems like a ridiculous question... Did you check your setup outside RealBand on mp3 or youtube? Both channels are ok for playback?

The reason I mentioned this... I almost destroyed headphone jack a few days ago. So when stuff like your happens, that I do not understand, I usually follow this protocol.

1) Close out of all programs, listen to plain mp3 on windows media player. If unhappy stuff is still happening, I would try to restart PC. Usually that solves step #1

2)If step #1 was successful, I would open "troubled" software and look closely at mixer balance?... If mixer does not solve it try external interface (UMC204HD, I assume) proprietary software driver setting, if there is an issue with L / R channels?

Let us know if you figure it out.