Hey there Kayy50,
It sounds like you've worked to isolate the issue to your iPhone. To be sure, try testing another Bluetooth audio device with your iPhone and compare the sound quality. Test in another location and compare the results to isolate away from possible interference.
Check out further troubleshooting in the following article: If your AirPods Pro (1st generation) make crackling or static sounds - Apple Support
- "Make sure that you have the latest software on your connected iPhone, iPad, iPod touch, or Mac.
- Check that your connected device is nearby, and there's no wireless interference or obstructions between you and your device.
- Listen to audio from a different app to see if the app causes the issue.
If you still experience sound quality issues—or if the issues increase in loud environments, with exercise, or while talking on the phone—refer to AirPods Pro Service Program for Sound Issues"
If the issue persists, reach out to Apple Support directly to help investigate the behavior. You can use the following link to reach out: Official Apple Support
Kind regards.