If you have a mobile phone that supports audio recording, then it most probably stores the recordings in Adaptive Multi-Rate format (AMR). If you want to convert these to other formats (e.g. MP3) then I found that Mobile Media Converter by MIKSOFT works quite well. It also supports conversion of 3GPP video files to other popular formats including WMV and MPEG.
It’s available for both Windows and Linux but no sign of a version for Mac (yet).