In the SEQ window, under Edit to the right of each channel in the MIDI file, there is a Velocity command that can be used to lower the Velocity of every note on a track, this can correspond to lowering the volume.

You can also pick one channel and not have it play at all by unchecking the corresponding box next to it under the PLAY heading. That effectively mutes any instrument in the file.

But there is a certain type of MIDI file, "type 0" that you can run into by downloading free files from the internet, that type of file puts all instruments on one channel or track, making it impossible to see the listing as separate tracks. In this case, RealBand has a command that can 'Extract to tracks' by channels, then save as .mid file from RealBand, close realband and open the file again in bb. Or you might find much more control over MIDI files by using RealBand to open and play them in the first place. For example, any "type 1" MIDI file will open right up on separate tracks in RealBand, and there's a volume fader for each track in Tracks View. As well as Mute and Solo buttons.

--Mac