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The line needs to go in your config file after changing to your mounted drive and before launching the game like this:
mount C E:Games\

mixer master 20:20

cd titan

Only tested this on dosbox .74
This mix master command did nothing for Might and Magic World of Xeen for me.. Trying to record play with commentary, I have to scream to be heard over the music. If I turn the mic up any more it just clips out from over-volume.

I'm playing the GOG install of the game, using whatever DOSBox version it installs.