Recently I faced a problem with MTS files (AVCHD from Panasonic and Sony cameras) with audio getting out of sync after file edit.
MTS file plays fine with sound in sync (e.g. with Daum Potplayer). However after editing, sound is not in sync anymore. Searching internet I found my problem isn’t uncommon, but I didn’t see a definitive solution.

Note that I haven’t touched MTS files for several months; and in the past I haven’t had any sync issues. During the break I had Windows reinstalled, so the problem must be somewhere on my PC.

The issue shows up with some video editing software but not all (e.g. VirtualDub, Corel Video Editor have the issue). It seems to me it’s related to AC3 encoding of audio stream in MTS files. I haven’t found precise root cause, I guess some codec version or settings on my PC might be the reason; however I found several workarounds that suit my needs.

• BlackMagic DaVinci Resolve doesn’t have an issue with MTS files. So I can do color correction in Resolve and then save file with an in intermediate codec (e.g. CINEFORM) for editing with other video editing software. Once MTS file is converted to another format, the issue is gone…
• I can convert audio to another format using AVANTI/ffmpeg. Copy video stream as is; and convert audio to PCM for example – then the sync issue is gone, file can be edited.
• VSDC Free Video Editor doesn’t have an issue with MTS files. The basic version is free. Unfortunately the latest update of Video Editor made the use of waveforms a paid option, but it isn’t expensive; or you can use an older version which has waveforms enabled for free.