It might need a motherboard reset: Try removing the baterry, RAM sticks (if removable) and CMOS battery, let it sit for around 5-10 minutes and power it back on. PLeaso go to a local computer epair shop if you don’t know how to do it as you might damage internal components while opening it.
Try to unplug battery, charger and RAM and CMOS battery, keep the power button pressed for 20 seconds, plug ONLY charger and see if it beeps, with no RAM installed it should beep. Seems similar as an no-post issue I think.
I've encountered this issue with all the iPhones I've had. It's an iOS issue, I think it can't stream 2 audio streams at a time. Swipe to open control center and play the music. Then turn off the sound in the game using game options.
You should try cleaning all the ribbon cabble connectors and try cleaning the lightning connector. If it won't work even after you done that you should try an ultrasonic tank cleaning. Good luck !