Hey, I like to listen to audiobook mp3's on my phone (GrapheneOS, Pixel 4a 5G) and hadn't had an issue. Then I dropped my phone and the screen wasn't responding, so I restarted it, and as it booted up, I got a screen saying it was "optimizing apps" or something like that.
Ever since, when I try listen to an audiobook, I get crackling sounds whenever the reader speaks, which is unbearable to listen to. I had been using the app "Voice" from fDroid to listen to the audiobooks, and I tried giving the app unrestricted battery usage in the background, but that didn't help. The crackling happens whether I use bluetooth headphones or the phone's own speaker. I also tried other apps, "Smart AudioBook Player" and playing the mp3s in VLC, but same issue persists (also given unrestricted batter usage). If I play a local video in VLC, I don't get the audio crackling.