VHS recording, sound issue

[Anoes]

Im am trying to record a VHS video and save as an mp4 file. The Video is about 56 minutes long. Everything works perfect until around 40 minutes into the recording, then the audio is muted. It appears on the audiometer, that the sound is still recorded, but when the file is saved and played the audio is lost from about 40 minutes into the recording. It is a very time consuming problem