How to post a song to Facebook
Being it's getting more and more difficult to create MP4 files these days, you need to know the alternatives.
Facebook has always been rather restrictive on what it accepts for media.
Where images are concerned, the site prefers static JPG and PNG images, but for GIF, sometimes it will accept it and other times not. If the GIF is static, then sure, no problem. If the GIF is animated, that's where Facebook gets picky. Sometimes it will accept an uploaded animated GIF while other times it won't, and I've never seen any uploaded animated GIF auto-start (you have to do one of those hover-over-to-animate things.)
Where video is concerned, MP4 is the only thing accepted as far as I can tell. I've tried WMV a few times, and I've never been able to upload one successfully. And as far as whether Facebook accepts MOV files, I don't know because I've never tried.
Where audio files are concerned, Facebook doesn't accept those, never has and probably never will.
The only way to share audio on Facebook...
...is by hosting the song somewhere else.
The two best options I've found for free audio hosting are BandCamp and SoundCloud. My BandCamp page is here.
With either service, what you do is upload your song, wait for the site to process it, then go to the page where the song is, look for the share button, click that, and there will be an option to share to Facebook. Then you just share out your song. Simple and easy. With either BandCamp or SoundCloud, the process is more or less the same.
Which do I recommend? That depends on what you're looking for.
If you want the absolute easiest possible way to share your song to Facebook, SoundCloud is the better of the two because their "share" button is easier to work with.
If on the other hand you want the easiest possible way to sell your song, BandCamp is the better of the two. Not quite as easy as SoundCloud for sharing to Facebook, but better to make it easier for your fans to buy your song.
Can you post audio to Facebook in MP4 format?
Yes. Technically, this is uploading video even if it's just a background image with an audio backing track, but yes, you can do it.
The only roadblock here is that it's not exactly easy to create MP4 files for this purpose.
For regular video, sure, that's easy. Just point your phone at something, record the video, done deal.
For video that is image combined with audio, that takes a lot more effort. If one were to do it the free way, the easiest (and I say that very loosely) way is to concatenate the audio and image files using FFMPEG. I know how to do it, but unless you know how to use the command line, I don't recommend this method and to just use BandCamp or SoundCloud instead.
One more thing...
Where the song recording process is concerned and what to do with the audio file afterward, the free Audacity software and the standalone Zoom R8 recorder both export WAV files. I know that BandCamp handles WAVs very easily and never complains about that format. I have in the past had some issues with SoundCloud and WAVs, but that was a while ago and it might have been fixed by now. Either way, it is better to upload in WAV rather than MP3 whenever you can. The upload takes longer because of larger file size, but the end result is a better quality recording - which is much preferred if you plan on selling your music.