If you've ever seen a mixing desk, the faders range from negative infinity up to zero near the top, then usually up to about +10 so a channel can be slightly boosted too. XAudio2 works similarly: a volume of -10 dB reduces the volume, like moving the fader on a desk down. Directsound actually worked like this, but adding 100 to everything (so 100 became unchanged level, 0 became -100 dB). So an XAudio2 volume of -100 does in fact correspond to Directsound volume 0 (but XAudio2 can go even lower and do boosts too). I think I need to add a 'Set muted' for channels though...
Yeah, and be careful with really high boosts because it'll clip really badly and distort
To play a different music, call the 'Play music from file' action again. It should replace the music.