No *.mp4 video with MP3 sound playable on iPhones?

[mike23]

I sent a *.mp4 video to buddies.

Much to my surprise some of them can play the video but hear no sound.
I turns out that exactly these guys use an iphone.
Android users do not have a problem with sound.

So can it be that MP3 codec is not working on iPhones?

Which audio codec should I use instead which works on both: Android and iPhones?
I could re-encode the video.

[Asmodian]

AAC for iOS

[foxyshadis]

AAC is the only truly universal format, since iOS dropped mp3 audio in video files. (It obviously still works for the music & audiobook library.) You don't need to re-encode the video, just the audio, and remux.

Opus might work now, since they support it both as .opus and .mp4 in the music library since iOS 11. I haven't tested though.

[finlay31]

The format must be different.