Hi,
Might and Magic 3 should run without issues with Magic Dosbox. How do you install mm3 on pc? Via dosbox or it has windows installer. Is it GoG version?
In many cases is issue like this caused with incorect sound card setting. Almost every game has in game folder setup.exe, setsound.exe or similar program. Probably you need to update game configuration based on settings in mDosbox game profile.
Best regards,
Tony