I recorded some audio (my daughter singing), and wanted to send the audio to someone in a generic and friendly format, namely mp3. The Nokia handset records as an amr (Adaptive Multi Rate) file. A good player, like VLC will play this quite happily as-is, but a lot of people wouldn't know what to do with an amr file.
Ubuntu talks happily to my phone, so getting files off is easy. I have a USB data cable (CA-53) for the phone (Nokia 6234). Ubuntu mounts the micro SD card automatically, if you choose "Data mode" from the phone menu once plugged in. "Default mode" doesn't work. Depending on how your handset is setup, you may have to use the handset interface to copy stuff from the phone's built-in memory to the memory card.
I found a program called Mobile Media Converter, which does a great job. I simply downloaded the archive, extracted it somewhere and ran the executable from there. The interface is straightforward, and even supports drag'n'drop if you're into that sort of thing. No settings to tweak, it just works. It's based on ffmpeg, which you may or may not already have installed, but comes with it's own version, so should work regardless.