The TD-15 can accept stereo .WAV files that are 8, 16, or 24 bit with a sample rate of 44.1kHz and/or mp3 files (44.1kHz 64kbps-320kbps).
If you're sure you have a .WAV file or mp3 file that match this criteria and it still won't play in the TD-15 , then the file may have embedded metadata, or "tags," which will prevent it from playing. This can happen with files that come from DAWs ("Digital Audio Workstations"), other audio programs, or sample libraries.
You can strip this extra information from the file by converting it to Apple Lossless or FLAC format. After removing the information, the files should play in the TD-15. You can use various free programs (iTunes etc.) to perform this conversion.