MP4 on iPad does not show if audio is MP3?

[David22]

Why would a movie i encoded to MP4 but set the audio to MP3 not even show up on a iPad? Change the audio to aac and it shows up with the same video settings.

I only picked MP3 since i thought it may reduce the file size.

[Cornucopia]

Mp3 is compressed audio as a raw, un-containerized stream. Called "*.mp3"
The equivalent for its successor can also be raw, un-containerized, but that would be called "*.aac" or occasionally *.adts. When in an mpeg container, it goes by the container name. Thus: *.mp4 or *.3gp for std industry containers (whether with or without video), but also *.m4a, *.m4b, etc for Apple-centric containers (when without video).
You have an ipad. Aka Apple product. They are going to have stricter expectations in their supported formats.
Could be they only expect avc + aac in their mp4 stuff on that device now (they barely support mp4 part2 video anymore). They have also been strongly discouraging mp3 in favor of aac (due to lack of drm capability with mp3), which may be evident in their changing what's acceptable in their video products as well.

Btw, aac is much more efficient than mp3 so re-encoding doesn't make sense unless you were drastically reducing bitrate (and thus even more drastically, quality).

Scott