I am currently using the Trial edition of Premiere Elements 2021 on a Dell G3 laptop with 16 GB RAM, SSD drive, Windows 10, etc. The program generally runs quickly, but I am seeing a sync issue with some recordings from our online church service.
Several MP4 files play well inside and outside Elements, but a couple are out of sync between audio and video by quite a bit. Strangely, those same 2-3 minute clips play fine in Windows Media Player, Power Director 15, or Windows Photos app.
MP4 is a wrapper, what is inside YOUR wrapper - Exactly what is INSIDE the video you are editing? Report back with the codec details of your file, use the program below Free program to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download - when you analyze your file in MediaInfo and post a screen shot in the forum, do so in TREE view
My GUESS is the offending files were recorded VFR (Variable Frame Rate) which does not edit well