So, I found these instructions the other day after being in a group where one person was really, really loud, and another was almost too quiet to hear.
I tried it out last night on the raid and it worked beautifully. Every single person who talked last night was at almost exactly the same volume.
So I figured I'd better post these instructions here, considering how friggin awesome the end result was:
1- Go to setup in ventrilo.
2- On the top right side, check "use DirectSound" for the output device.
3- Click the SFX button.
4- Select the "Compressor" sfx item in the list and click the add button.
5- On the screen that pops up, set the options to the following (note, the number in the middle of the slider is the current setting):
Gain = Adjust for how loud you want people to be. (I use 15)
Attack = 0.01
Release = 200
Threshold = -30
Ratio = 100
Pre delay = 4.0
6- Select ok on each setup screen to save your changes.
7- All done.
Each person on vent should now be output at roughly the same volume.
If the volume is too quiet or too loud, the sliders on the main setup page will no longer modify the volume in a significant way, you'll need to go back into the SFX config and change the "gain" parameter as appropriate.