Lost audio when FF MP4 files

I created MP4 files from family VHS tapes. I used handbrake and then movie maker (windows). I loaded the mp4's onto new portable Seagate HD (USB3.0) and then plugged HD into blue-ray player for playback on TV. The mp4's are between 2-9gb in size. I can play the files but if I fast forward and start playing again I lose the audio. I've tried pausing it to see if audio comes back but it doesn't. Are my video files to big? Any other suggestions?

 

Well I used movie maker because there was some small editing that I wanted to do prior to making final mp4. For instance there was bad footage from original VHS tape ie. snow, that I wanted to edit out to make it better viewing.

I'm doing this for the in-laws and purchased them the portable HD already. They had the blue-ray (LG purchased in 2011) player which says it can play mp4's and many other formats. Would just like to find a work-around that wouldn't include purchasing something else. I've never worked with Roku but can see I could get them one for $49, but then I'm not sure about their wireless strength at TV (it's not smart) so then I can foresee me buying them wireless range extender/router. This has turned into much larger project than I had envisioned lol.

Thanks for help in advance.