This seems terribly stupid, but I'm really unable to find a solution: I reconfigured Screamer to use MIDI instead of CD Audio, however the volume is set too low and the normal volume control you use in XP isn't available anymore.
I'd be grateful for any ideas.

In Vista MIDI is linked in with the global volume controls, your best bet is the turn the global volume up if it isn't up all the way already and turn your speakers up as well.

Dang. that won't be any good as the motor sounds will simply overshadow the music... but maybe I can get Vista (or 7 as is actually the case) to use Timidity, which as far as I remember can control the volume directly.

If you could seperate the music from the game or get the games music you could run it in something like winamp while playing the game.

This is definitely a solution and I'm going to mark it as such... however I'm still trying to use a real synthesizer and Timidity is working fine... except it won't accept the amplification option :(
Edit, I'm routing it out to WIndows and then on to Timidity... how can I control DOSBOX' internal mixer?

By using the MIXER command.
Entering plain MIXER gives you the current settings, normally everything is set to 100:100/+0.00dB:+0.00dB.
The "Channels" you see can be used as parameters, for example entering
MIXER SB 50
sets the Sound Blaster volume to 50(%?), attenuated by ~6dB. I'm assuming this is for digital voice only because there is a separate entry for FM synthesis. But since you're using Windows MIDI, it shouldn't really matter either way.
You can experiment with the command by placing it before starth.exe in the [autoexec] section of dosboxScreamer.conf.

Thank you! That's exactly what I was looking for!

Dude you rule so much. Now I can listen to my own music properly whilst driving. :D
Thanks!