You can tryusing this free software MIKSOFT Mobile AMR Converter
*www.miksoft.net/mobileAMRconverter.htm
or else there is this great software which we all know, i.e. QuickTime Pro (remember the Pro version not the free)
*www.apple.com/quicktime/download/
In QuickTime you can export AMR files to WAV format.
Both of the softwares are extremely easy to use, but of course QuickTime scores a little bit in termsof speed.
Then there are umpteen softwares to convert files from WAV to MP3 format.
That's it!!!