This took me a while to figure out but this happens with calls on Messenger and rarely sometimes on Phone app. After the call has ended I would listen to music and things would be very quiet. I thought it was an issue with my Bluetooth headphones so I check the app and then it says I'm unable to change some settings cause I'm currently in a call. Only way I fix this is if I restart my phone or force stop Messenger.
Di your head phones switch to a "headset" profile when making a call?? They might not be switching back to headphones after using the headset for a call...
