SanDisk Clip Sport/Jam supported file types includes AAC (Apple iTunes AAC-LC (.M4A) Audio)
according to this page —> http://kb.sandisk.com/app/answers/detail/a_id/14130
My Clip Jam will play files created by neroAacEnc, but it won’t play files encoded/muxed by FFmpeg.
I’ve raised a trac ticket with FFmpeg, maybe they will be able to sort it out.
Here —> https://trac.ffmpeg.org/ticket/4998
