I noticed mplayer was using the sun output driver. First I would try using oss instead:
$ mplayer -ao oss somefile.wav
Failing that, I would suggest using mplayer to resample the WAV files since they seem to be the problem. Since scaling the playback x0.1 worked, try this:
$ mplayer -speed 0.1 -srate 8000 -ao pcm:file=somefile-resampled.wav somefile.wav
That will scale back the playback rate and keep the sample rate at 8000Hz and write a new file based on that. If it works, I'm sure you could write a small script to batch convert all of the files. I would try playing one of those new files back on your Windows box and see if it sounds right.
Keep in mind, WAV files behave a little differently than mp3's so an mp3 might be fine while a WAV file may sound messed up. Let's find out whether your files are the culprit or if it is a driver issue. I found a file very similar to the type you have as far as sample rate goes (8000Hz 16bit PCM). It should sound like a normal person talking. Try downloading this audio file and see if it plays correctly:
http://www.nch.com.au/acm/8k16bitpcm.wav. If that doesn't sound right, then it probably isn't your files.