I have 3GA files which were produced by a mobile phone.
To convert 3GA files into MP3 files, I use:
$ avconv -b 256k -i OneOfThe3gaFilesIRecorded.3ga PreferredMP3.mp3
For example, a 3GA file of 595KB can become an MP3 file of about 1.7MB. I like MP3 files because many devices can play them.
Update: 5 August 2017
People make many videos every day. I wonder if people still use these audio formats. If they do, MP3 is still a useful format.
iPhones and Android-based phones are usually equipped with cameras. They don't just record the voice or sound. For playing videos on a computer (Windows, Linux), WebM may be a good format. I wrote another small post to talk about WebM.