Thank you for posting in the Apple Support Communities. I see that your iPhone 8 Plus sounds muffled when not in speakerphone.
It sounds like this is happening primarily when on calls; is that right? If so, you'll find some great steps here If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone :
I hope this helps.Check the connections
If your iPhone is connected to a headset, you won't be able to hear through the iPhone receiver or using speakerphone. Follow these steps, testing after each.
- Make sure that you don't have anything plugged into the headphone jack or the dock connector.
- Go to Settings > Bluetooth and turn off Bluetooth.
Clear the receiver
The iPhone receiver won't work properly if it's blocked or dirty.
Follow these steps, testing after each:
- Make sure nothing is blocking the receiver, such as a case or screen protector.
If you have a new iPhone, remove the plastic film on the front and back of the device.- Check the receiver opening to see if it's blocked or dirty. If necessary, clean the receiver opening with a small, soft-bristled brush. Make sure that the brush is clean and dry.
- While you're on a call, turn on speakerphone. If you still can't hear, then your network or reception could be the issue. Try calling again later or from a different location.
Restart and update
- Restart your iPhone and test again.
- If there's still no sound or poor sound quality, update your iPhone to the latest version of iOS.
Warm regards.