It is not the bitrate of the file but the sampling rate that has few options. Both the bitrates (expressed in kbps) are supported, but the .wma is definitely NOT.

The sampling rate MUST be either 44.1kHz or 48kHz.

And you are already probably aware, a 56kbps or 96kbps MP3 is going to sound pretty terrible... are you SURE you can't find them encoded at a better rate than that?

One last thing to check for is... is there any unusual characters in the name of the files? p.51 of the manual shows what can and can't be used..

Hope this helps.