To install the app in a different device (smartphone or tablet) in full(premium) version you need to log in to Google Play using the same account you used for purchase (the same user -email account). The Internet connection is necessary as MP4Fix runs for the first time.
More info: https://mp4fix.com/question-and-answer/reinstall-mp4fix/
Hi Farm Hillside,
I'm sorry your audio was off.
Sometimes there is no audio data in MP4 video file (e.g. where other application blocked your microphone). When there is no data even MP4Fix can't fix it...