No audio from MP4

When playing MP4 videos there is no sound. There is sound with other types of videos. Anyone one know why ??

It is impossible to say for sure based on the information provided.

An MP4 video file is normally expected to contain an MPEG4, AVC (H.264), or HEVC (H.265) video track along with an AAC audio track. However, since you do not indicate how the the file was sourced/created or confirm what kind of data is actually stored in the file or which player/device/OS is being used for playback, it is impossible to know if the file actually contains an audio track or, if it does, say whether or not the audio is playback compatible. I.e., all "normal" MP4 files should be audio playback compatible with any current (and most older) media players/devices. If it isn't, then you need to examine the file further.

Basically, you need to open the file in your media player (or a media utility) and check the "Inspector" or "Media Information" window to ensure an audio track actually exists and, if so, what kind of audio data it contains. If you're unable to do this, then post a sample file for download by others here for further examination.

I am somewhat technologically proficient, but I do not know how to check which format of mp4 I am dealing with. I can tell you there is no sound on my iPad nor iPhone, but there is no problem heading the audio on my PC. Does that help ?

If by "heading" you mean "hearing," then it may help—depending on the video player you are using on your "PC." As previously indicated, iOS and macOS devices expect MP4 files to contain AAC compressed audio. If you are able to hear the audio on your PC, then the most likely reason would be that the MP4 file either contains a different audio compression format or the AAC audio was compressed using incompatible settings. This is why I recommended you inspect the file contents using a media player or dedicated utility app. See MP4 examples below:

The MP4 extension refers to the file container only—not the data actually stored in the file. Apple products are very compliant when it comes to playback standards and would normally prevent you from creating an MP4 file which contains incompatible audio. Basically this means users can store other forms of compressed audio in an MP4 file but whether or not the audio will play depends on the app used for playback and which audio formats it supports. While the description of the compressed data varies to some degree in the above sample images, each app does indicate audio data in my file is compressed as AAC (MPEG-4) which is playback compatible all macOS, iOS, tvOS devices, as well as, the media player used on your PC. Now you need to check your file and see if it contains some different form of audio compression or employs non-strandard/incompatible AAC compression settings.