This is the old iPod model from around 2002, that has a FireWire port instead of a dock connector, correct? This is the tech specs for the 2nd gen iPod.
"Audio formats supported: MP3 (up to 320 Kbps), MP3 Variable Bit Rate (VBR), WAV, AIFF, Audible® (for Mac only)"
However, that seems to be true only if the iPod has its original software. This document says additional audio file types are supported, with later versions of iPod software.
The 2nd gen iPod can be updated to software version 1.5, I believe.