In the past, I converted mp3s to .wav format using a free tool (one of dozens) found on the internet. The tool I used was for Windows, but I just did a search and found http://www.mpg123.de/, which can apparently do this using mpg123 -w <filename>.wav <filename>.mp3

Wav files contain PCM data in an uncompressed (RIFF) format which makes them easy to read and manipulate. PDL has a module PDL::Audio that can read .wav files directly: $pdl = raudio "file.wav";

One thing worth mentioning is that mp3 and ogg formats are "lossy", so the fidelity of the sound it manipulated. Human ears may not detect it, but FFT certainly will.