The instructions for getting dosbox to play sounds through an external softsynth are pretty much the same everywhere you look:

http://ubuntuforums.org/showthread.php?t=618914

I've followed these instructions, and it does not work. It says 'client initialized [128:0]', and it plays MIDI, but it sounds exactly the same as before. There's a world of difference between dosbox configured to play through timidity and dosemu actually playing through timidity.

On a whim, I changed my config like so:

[midi]
mpu401=none
mididevice=none
midiconfig=

It still plays, and still sounds exactly the same! It's like dosbox 0.73 is completely ignoring this section of its config file.

EDIT: The problem was in my app's settings. It was set to use the SB for music instead of general MIDI.