SlimServer users have reported problems with playback of WAV and AIFF files that are anything other than 44.1kHz and 16bit. For example, AIFF files at the 48kHz or 22.05kHz sample rate play back too fast or too slow, while WAV files at the 48kHz/8bit setting have trouble playing at all.
If you experience these problems, try using a music editor to change the sample rate to "44.1 kHz" and the bit dept to "16 bit". Or, use iTunes as your music server, which does not appear to have these playback problems.