Hi bilbonorway,
Thanks for the post in Apple Support Communities! If we understand correctly, your caller's voice is muted when there's a sound on your end. Is that right? We're happy to see if we can help out with this.
For verification, does this only happen on FaceTime calls, or does it happen with phone calls as well?
Does it happen if you try using headphones with your iPhone? Knowing the answers to these two questions will help in narrowing the issue down a bit.
Also, are you able to replicate this on every call, or is the issue intermittent? Does it help if you Restart your iPhone?
If not, feel free to reply back here with the answers to the above. Be sure to include your iOS version as well, found under Settings > General > About.
Best wishes.