Hey curlybrian,
We'd be happy to help out. It wouldn't necessarily be unexpected for some muffled audio if you're not facing or speaking directly towards the internal microphone of your MacBook Pro. Do you only notice this within FaceTime or do you also notice this if you try to record a Voice Memo? If you haven't yet tested this, please do so as this can help with isolating the matter further and help us determine if this is specific to phone calls or FaceTime calls.
Your mic is located in the upper right portion of the keyboard and speaker section of the computer for your reference.
We'd also suggest checking to see if you can use Mic Modes while in FaceTime using these steps:
Use Mic Modes on your Mac
You can adjust the microphone settings on Mac to filter out background sounds or capture the sounds around you.
Note: Mic Modes are only available on Mac models from 2018 and later.
1. On your Mac, open an app that supports Mic Modes, such as FaceTime.
2. Click Control Center
in the menu bar, then click Mic Mode.
3. Do any of the following (options vary by app):
• To filter out background sounds, select Voice Isolation.
• To include the sounds around you, select Wide Spectrum.
• To revert to the standard mic mode, select Standard.
This is outlined further here: Use Mic Modes on your Mac
We hope this information helps out.
Take care!