@CHILLN I tested https://x.com/0xmitsurii/status/2007165337587069017 in Opera GX 125.0.5729.58 64-bit on Windows 11 23H2 and it played no problem for what it's worth.
What does https://html5test.opensuse.org say for h.264 and aac support?
Any difference in a test standalone installation?
Does Vivaldi have the same issue?
Opera relies on the Windows Media Foundation for proprietary codec support, so it could be a Windows issue that was caused by a recent Windows update.
Download the zip file linked on https://dvdloc8.com/clip.php?movieid=12167&clipid=1 and extract the mp4 from it. Does that mp4 play in the "Media Player" app in Windows? Does it play in the "Windows Media Player Legacy" program? Does it play if you drag it into Opera, drop it on Opera's tab bar and click play?