Volume and sound randomly skipping - how to diagnose?

When listening to media on my phone (Android 11) for the last week or so, the audio will change at random and unprompted, e.g.: - rewinding or fast-forwarding (NB - these functions are mapped to my volume keys if that is relevant) - volume going up or down (including down to zero volume, and up to max) - media pausing at random and also playing at random (this is the biggest issue, as even if I have force-quit my media application, any time the phone is turned on, the application randomly restarts and may start playing my most recent audio at any volume.) It does not happen in a predictable way - sometimes behaviour is normal for hours at a time, sometimes there is a glitch every few seconds. It does not seem to be related to network issues or my headphones, and the volume change buttons don't seem to be obviously broken (and also shouldn't affect pause/restart functions). Without touching anything, I have also occasionally seen the volume slider oscillate randomly up and down. This happens across several apps. I cannot tell if it is a hardware or software issue. Is there any way to either diagnose the cause of this sort of problem, or any way to lock the volume or pause-play functionality so that it does not change unless I explicitly confirm it?