[Zubersoft]

I don't really think you can do that (if I'm understanding you correctly). I'm not sure exactly what sound you are wanting to hear through the headphones, but if it's sound generated by the tablet (like the playback of an audio file or the metronome), those can't be routed to your keyboard unless it's making itself available as an audio output device through USB. If it is doing that, you may just need to go into the main tablet settings (outside MobileSheets) and ensure that USB audio routing is enabled.

MIke