Please help - MPG files with no sound on Mac

Greetings Apple community -

I have back ups from an iMac that I no longer use, stored on a portable hard drive, which I am accessing via a more up to date Apple MacBook.

I am going through a large collection of media files, mostly jpegs and MPGs. I can view all of them, but I notice that the MPGs play via Quicktime, yet there is no sound.

What could have happened to the sound - and how do I get it back?

Thanks in advance for your suggestions