The m4a format is basically the mp4 format, minus the video aspects.  In 2006, m4a had extremely limited capability with anything that wasn’t iTunes or an iPod/Mac.  The mp3 was king.  11 years later, the m4a format has far, FAR greater compatibility.  It’s also the superior lossy compression format.  An m4a file at 256kbps will sound EXACTLY like an MP3 at 320kbps.  This means we can upload a 320kbps m4a and have far superior sound quality to the 320kbps mp3, or we can upload a longer piece at 256kbps m4a while still remaining within that 16mb filesize limit.