Even though mp3 format recording isn't available in mainline Asterisk, due to the significantly smaller file size various methods to convert after-the-fact are available via a quick Google search.
Attached is a small +- 4 line patch that does one small thing. It modifies cdr_audio.php to identify mp3 files (by extension) and give the correct mime-type for playback in the browser.
Regardless if mp3 support is added for recordings in FreePBX, in any capacity, this change will need to be included. Further, even though mp3 support isn't officially available, this change is so minor and will significantly improve integrations with the unofficial methods for mp3 recordings.