What type of audio files can be uploaded into the device?

RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 8000 Hz

You can use the free utility ‘Audacity’ (http://audacity.sourceforge.net/) to convert audio files into a format the device can recognize.

When you export an audio file with this program, you can save the output as “WAV (Microsoft) signed 16 bit PCM.”