Audio data can usually be described by rate, data size, data encoding, and channels. In order to get the proper encoding for Asterisk, you must provide the following details:

Mono, 8000Hz, 32K bitrate.

If you need to convert the file, you can use these tools:
- WinLame
- RazorLame

If running Linux/MacOSX, you can use lame application:

lame -q -p 
    --mp3input -a --preset 8 
    input-file.mp3 output-file.mp3