Maybe a couple of percent compression from common headers etc. but even uncompressed WAV files do not usually compress much in a "normal" archiver (though that's due to the characteristics of the file).
If the audio compression scheme is doing it's job well, every bit and byte in the file should be working too hard to be compressed.
Re-encoding?, well, even if the originals were all at 320k CBR, you could not halve the size and still maintain transparency - going from 320k CBR to LAME alt preset standard (the preset aimed at good quality, holding the size down as far as possible) would not save enough space - and if starting any lower, halving that is even worse.