mixer master 10:10

^put this in your dosbox.conf file (or whatever your config file is that mounts the drive).
place it between the drive letter, and the game.exe.
use a new line for this command.
i use 10 as the percentage level for the volume, but you can use whatever percentage, to suit you.
in your readme.txt file that came with dosbox, it says:
"MIXER
Makes DOSBox display its current volume settings.
You can change this way:

mixer channel left:right [/NOSHOW]

channel
Can be one of the following: MASTER, DISNEY, SPKR, GUS, SB, FM.

left:right
The volume levels in percentages. If you put a D in front it will be
in deciBell (example mixer gus d-10).

/NOSHOW
Prevents DOSBox from showing the result if you set one
of the volume levels."
if the decibel option doesn't work, use the percentage for the volume, like i did. took me awhile to figure all this out.