I recorded an mp4 file with 41mins, when i open it it shows the size and duration and plays through the file but doesn't have any sound. Does anyone know a way how to try and repair this or is this audio lost?
Try opening the file with some advanced video players just to find out if the audio is gone or there's a codec error.
Two video players that I use when having this sort of problems is Quick Time and Video Lan.
If indeed there's no audio attached to the video - there's little to be done - maybe useing a disk data recovery tool depending on the recording medium, to get at least some of it back (if it was written on disk).
I've had this happen before, howevr it was while recording the screen of my computer. Not certain if you've done the same or used a camera IRL. However easiest way to see if there's audio in a file is to play the file in VLC and press CMD+I or CTRL+I (Window>Media Information)
Notice under CODEC Details "Stream 0" & "Stream 1". 0 is your video and 1 is your audio. If you see Stream 1 (or 2) you have an audio encoded into your video. Next jump over to the "Statistics" pane right of "CODEC Details", you should see a lot of numbers in each section of that pane and the most important two parts are the top left "Input" and the bottom right "Audio". If you see that all jolly good then you probably just gotta give it some gas (gain). Many streaming / video players support this but if you'd like to further investigate, I suggest using Adobe Audition or Adobe Premiere (or both but Adobe Audition is strictly for sound and it a very high quality program). Best of luck!
I might be late but I've had the same issue - I tested a bit around and for me the solution was making the .mp4's audio track stereo. Don't ask me why, but for some reason Windows Movie Maker decided to not play 'mono' sound in mp4. (My) problem solved.
I hope some souls with the same problem will find my solution here (though the conversation is 3 years old already).