Hello stottt,
You titled your post as a sound issues with an iPhone, but also mentioned AirPods. So is the sound problem just with the AirPods? Or do you have the same issue with playing audio through the iPhone speakers?
If the issue is just specific to the AirPods, make sure you don't have a volume limit set. Go to Settings > Sounds & Haptics > Headphone Safety and see if the Reduce Loud Sounds is turned on. If it is, turn it off.
Use headphone audio level features on iPhone
If the problem is with the actual speakers on the iPhone, go to Settings > Accessibility > Audio/Visual, and make sure the Mono Audio option is turned off and that the Balance slider is set to the middle.
Aside from that, you'll also want to make sure you've covered the basics like restarting the device and making sure it's up to date.
Restart your iPhone
Update your iPhone
Cheers!