Please let me know how that works. I haven't tried XGM2 yet.
I have experienced the same issue with my games using the older XGM driver. Since XGM indiscriminately mixes in the music at full blast, I generally end up making new versions of my songs where the instruments are all just set to lower volumes at the beginning.
However, I'm at a loss for the correct solution here. I think the right thing to do is still to distribute the songs at full volume and let the application mix it as needed, since that's what you would do for any application other than XGM/SGDK, and XGM/SGDK happens to be the one weird, misbehaving use case. Or, put another way, if you're recording an orchestra and the recording is too loud, asking the orchestra to play more quietly is generally not the right way to fix it 🤷