Try to open the 'no sound' files in program VLC.
There is a good chance that the recorded audio is in the files, but some header data is missing and
Windows Media Player and m20d won't play it. But VLC will.
I also had 'stopped' recordings.
Issue was the SD card. MicroSD with adapter from Transcend, class 10.
It was at maximum class 5 and had reproducible write errors.
I replaced it with a reliable card an everything is fine now.