Why don't you support mp3 files?

The Public Switched Telephone Network (PSTN), which allows fixed line and mobile phones to call each other, uses the G.711 audio codec and, at the time of writing, is almost invariably sampled at 8KHz. Therefore, the best possible audio quality can be obtained by playing and recording files containing just that type of audio - G.711 (ulaw for the US, Canada and Japan, Alaw elsewhere) sampled at 8KHz. The wav format supports this, and that's why we use it. Playing and recording mp3 files would make the audio quality worse.