1.
edit dosbox.conf
Find:
mpu401=

The three lines starting with mpu401= should look like this:
mpu401=intelligent
device=alsa
config=128:0

Save and exit

2.
timidity -iA -B2,8 -Os -EFreverb=0 2>&1 &

3. Start dosbox

4. Enjoy General MIDI.

Note: You may have to occasionally change
config=128:0
To
config=129:0
in your dosbox.conf. Watch the output when you start timidity and it'll show which to use.

If you get permission errors, you may have to add "timidity" to the "audio", "pulse", and "pulse-access" groups in Users and Groups.