The extension 3GA refers to files of the type, 3GPP audio files. These are the audio only files which contain the audible content for the popular 3GP video files.

The 3GA file was developed as a part of the third generation partnership project which also created the popular 3gp video file format that is used extensively today across the mobile platforms for video sharing. Any mobile video player that supports 3gp files will also support the 3GA file and you can play the audio from such players. There are third party tools available on the internet to convert 3GA format to more popular audio formats like wav or mp3.