Is there a way to lower the music volume without lowering the sound effects?
Which game(s) are you talking about, and with what sound sources (e.g. Ultima Underworld using Rolant CM-32L (Munt) for music + sound effects, and Soundblaster for sampled speech)?
If you are using Soundblaster for both music and sound effects, then I don't think there is a way to change the volume balance (as the game has no internal mixer).
If you are using Roland + Soundblaster combo for Ultima Underworld (the most best sounding combination), then all the sound effects, as well as music, is generated by Roland. Soundblaster is used only for the digitized speech during the intro (and where else there is speech in the game, maybe in the ending?). Since the game itself doesn't let you change music/sound effect volumes separately, you can't make Roland music louder or quieter compared to Roland sound effects. The volume balance between music and sound effects is what it is, as intended by the game developers.
But if that UUW intro speech is too quiet compared to the Roland music, then read on.
Ultima Underworld 2 though uses digitized (Soundblaster) sound effects too so there you might want to rebalance the music/sound effects volumes. So what you want to do is to somehow make DOSBox Roland MIDI output quieter, and/or Soundblaster (digitized) louder.
If you are using Munt (Roland MT-32 emulator), are you also using BASSMIDI or VirtualMIDISynth with it, or have you edited dosbox.conf instead to get the correct MIDI output for DOSBox? I'm asking this because it may be BASSMIDI/VMS affect the MIDI output volume as well, IIRC BASSMIDI had a slider for the MIDI output volume, and VMS has some MIDI volume shortcuts explained in the "Shortcuts" tab. But then I don't think BASSMIDI works fully in Windows 8.1 or 10, so BASSMIDI/VMS may be useful only in Windows 7/Vista...
Another way may be using the DOSBox mixer settings. So edit the game's DOSBox config file, and at the end of the file under the [autoexec] section, add a line for MIXER. I don't think it has an option to lower MIDI volume, but it has an option to raise Soundblaster (or master) volume. https://www.dosbox.com/wiki/MIXER
I tested with it a bit, so if I added e.g. a MIXER line into dosboxULTIMA1_single.conf so that the [autoexec] section looks like this (I added the bolded
line), then at least for me the intro speech got louder compared to the Roland MIDI music. I think the mixer setting 100 is the default, so over that makes it louder than the default (also compared to MIDI music).
# Lines in this section will be run at startup.
mount C ".."
imgmount d "..\game.gog" -t iso -fs iso
MIXER MASTER 150:150
But as said, this changes the volume balance only between sampled and MIDI sounds. In the first Ultima Underworld you can't change the volume balance between other sound effects and the music, as they always come from the same sound device (either the Roland MIDI device (e.g. the Munt emulator), or the Adlib/Soundblaster device emulated by DOSBox).