The TD-50's Song section can play stereo .WAV files that are 16 or 24 bit with a sample rate of 44.1kHz and/or mp3 files (44.1kHz 64kbps-320kbps) from the SD card.
If you're sure you have a .WAV file or mp3 file that matches this criteria and it still won't play in the TD-50, 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, and then convert it back to the correct format. After removing the embedded data, the files will play in the TD-50. You can use various free programs (iTunes etc.) to perform this conversion.
http://www.rolandus.com/support/knowledge_base/201977529