It's a tough one and there might be several solutions to this.
If he's from a decent european country there are strict rules about money back, no matter if you find bugs or not.
1. Is the audio a software problem or could it be an external player problem? It sounds odd that the audio is out of sync with the video.
2. If you post a link to his sales page then it's far easier to help you, but IF they have made claims (visual or text) then they should honour that. If they have tried to sell you something based on false promises then get your money back.
I know it's not much help but it's all I got
Mike