What was the command line you saw?
The command line arguments and console commands (or variables) are a different thing, though some console commands can be used as command line arguments.
Since you're playing original version of Quake II, I assume you are using the original
.exe and not modern port. Here's an old page that has console commands and command line parameters:
https://www.eecis.udel.edu/~portnoi/quake/quakeiicom.html
And I don't see anything related to the music volume in that list. There is only s_volume, which is for sound effects. Original Quake II had music implemented as playing the audio tracks from the CD. Which if I remember correctly was an analog signal coming via 4-pin cable from the CD-drive to the Sound Card.
So try lowering the CD volume slider a bit in Sound Mixer in Windows. The original game has no music volume in the options due to that reason.