No audio and slow videos after battery replacement

Hello lovelies,

My phone is OPPO AX7 (CPH1903), with Android 8.1.0.

I bought a new battery, same model and parameters as original. My brother was the one mostly doing it, as he has some experience already. We followed a video of someone doing it in the same phone, unscrewing the motherboard and gently lifting it a few degrees to access battery port. The one thing that was disconnected was touch detector, which works fine after re-attaching it.

My problem started after that operation though, but it behaves more like a software problem.

The sound from main speaker only comes out as silent cracking, like some campfire. Same thing when connecting headphones. It doesn't apply to calls, the sound is fine in them whenever they're phone calls or in-app one. I can hear and talk as usual.

However, there's problem also with videos. If i try to watch any, not only there's no sound, but also the speed is >2x slower. (again, doesn't apply to video calls and discord streams, apparently.) I can't record videos, the recording is unable to be finished.

Last thing which is weird for me is rhythm games being affected, being as slow as videos, while other games are unaffected.

I hope there is a fix that won't need my phone wiped clean nor pricey service, but I'll be glad to have any clue at all what's happening. Thanks in advance!