MP4 files in firefox query

Hi folks
I'm playing around with a local web server - testing locally

I'd like multi-media content to be played directly in a web browser instead of either first downloading the file or having to use an xtra 3rd party app.

Seem to have a problem with .mp4 files -- the ones from the BBC all play properly with sound etc in firefox (all local files so testing on the local server via localhost.
Other mp4 files just give no sound ????


Example :




All the ones from the BBC for example Neanderthals etc plays properly with sound.

Star trek TAS (Animated series) plays OK but no sound ????

I'm trying to get this to work so eventually users can just logon and play with a bog standard web browser and don't need to download and use some type of music player.. I know mkv files and HEVC 265 need some extra handling but that's another issue.

Does Chrome or even Edge work any better for this

Cheers
jimbo

Do they play sound in VLC?
And in VLC you can get the audio codec information, see what is different about the ones that do not play, see if the files that don't play have a supported codec installed in windows, like maybe you need some codec pack installed.