For anyone who wants to replace the existing music files, you can use WAVs, or OGG Vorbis.
If you use OGG files (my preference because of the compression – the default music is OGG) you have to rename the extension to .wav
128 kBits, VBR, 44100 Hz. Both stereo and mono seem to work.
You can easily convert MP3s to OGG with a program like DB Power Amp or Sound Forge.
Try using Saving Private Ryan/MOHAA/Band of Brothers. Believe me – if you think the game is tense as is, you should hear it with some of that MOHAA music. Makes the hair stand up on the back of my neck.