It is a serious answer.
My article is deleted every time automatically once I link other unanswered articles to it. No matter how much "bad publicity" I remove, I always get it removed by "rants or non-constructive comments".
When you take the phone away from the ear, as little as 2 inches, the noise canceling system catches your voice through the mics at the top of the phone as much as the lower mic, considering your own voice a "background noise".
There's no solution for that. An Apple Engineer told me by phone "Apple designed it so" (you having to chose "speaker" even for a still-close quick look to the screen)
If you keep the phone against your ear, your voice (sent to the listener at the other end of the call) won't be muted.
I answered that because the OTHER cause you CAN fix it: avoid ANY slight stroke or rubbing to the camera mic (little hole). If you test it with another phone and BARELY stroke the camera with your fingerprint, or hair, your voice will be muted for a long second. Keep a pieace of clothing or bijouterie or hair rubbing against that little hole and you'll get your voice permanently muted.
Stoutie: Your idea of a joke would work if the issue was "i can't hear others" or "i can't hear myself", but the OP clearly stated his voice sounded like that with different callers, so its obvious we are not talking about hearing his own voice in his own phone but other people listening to his voice.