Thank you for your help so far. I appreciate it!
In game, there is indeed a slider for "BGM" (The midi background music) But it literally has no effect whatsoever. I can't even mute the sound in game!
In the "recording" tab, there is indeed a "Stereo Mix" device. It was disabled and I enabled it. I fiddled around with it's properties, and the only effect I've been able to produce is an "echo" when I max out the volume slider under "Levels" tab and check "listen to this device". This echo effects every sound, not just midi. Setting the volume slider to 0 does nothing to lower the midi volume.
I'm at a loss... O_O