DOSBox does not control MIDI volume, but some games may have a built-in volume control that changes the volume by changing the MIDI data. It depends on the device providing MIDI, but you should be able to find a volume control in Windows.

DOSBox can control the volume of CD audio with disc images mounted via IMGMOUNT, and possibly other types of mount methods that extract CD audio as data (not sure about that), but for many of the other mount types you again will need to use a volume control in Windows.

Think CD/MIDI audio level is controlled by host.

That's not very fortunate, because I'm not able to adjust CD volume in Win 7 on its own... But I'm using .cue image in Dosbox, mixer should be working, I don't get it.
And Midi - could the problem be Little Big Adventure using Roland MT-32 or Wave Blaster on SB16, not common General Midi? I can adjust Midi volume for example in Duke3D or Doom.

are you sure you are first mounting the cue and then trying the mixer control ?

(mixer control from the commandline in dosbox, not the one in the game)

I must apologize, mixer works for CD audio. Just D-fend Reloaded's mixer doesn't work. But Midi is still problem for me 🙁. Is there any way to adjust midi volume directly in Win 7?

Maybe with hardware midi synth, not with software synth in Vista/7. AFAIK

You could try setting DOSBox volume in Windows mixer to a value you like and then adjusting the mixer for SFX in DOSBox I guess.