Along with the above mentioned programs, the ones i've tried are AUDIO COMPOSITOR (shareware...) and the one i would recommend TIMIDITY++ (open source, FREE, and for most platforms - WIN, MAC, LINUX...)
Audio Compositor was processor intensive and even with the highest settings (also tried the lowest to compare) i got pops and crackles in the final .wav output.
TIMIDITY++ on the otherhand works flawless for me. I can output to .wavs or in 'real-time' output to your sound card! I've experienced no pops or such with either output method, BUT installing and configuring the program can be hard for *some*.
Get the win32 GUI version: (i assume your running windows)http://www.stardate.bc.ca/eawpatches/source/tim-gui.exe
And eawpats file (30 Meg of samples NECESSARY to hear anything) - 22Meg download though... :http://www.stardate.bc.ca/eawpatches/eawpats11_full.rar
PLUS winrar to unpack the above file.
You'll need to read the documentation for the install, BUT it is easy
If you have problems leave a message and i'll help you configure it...