Since putting on a lifeproof cover for my iPhone 5 people can't hear me on the other line

an extra set of holes for speaker. May be plugged partially with ear wax build up. Try holding over steam for a few minutes and wipe lightly with cue tip

open the dock connector port, then with a quarter pry open at the small slot on the bottom left of the case.

Does it recognize it when the life proof case isn't on the phone? If so there isn't a way to get it to recognize it when the case is on because the case is to thick unless you but some kind of extension for the 3.5mm jack on the phone.

One of the internal connections likely broke or came loose. Did you take it to an Apple store? They should be able to diagnose it and tell you what it will take to fix it.

This sounds like the earpiece has gone, you should be able to hear them if you put it on loud speaker or if you are using bluetooth. However as it is only 5 months old it is covered under warranty so my advice would be to take it back to where you bought it or contact apple direct. They will repair it for you however you will be without a phone for a number of weeks

you probably blew the earphones

sometimes the phone thinks there are earphones plugged in try plugging earphones in then unplugging again also try volume up and down and it will display earphone on screen keep plugging in and out until it does not say earphone on volume
if that doesnt work restart phone

I had a similar problem....it was usually after I unplugged the headphones....the phone thought they were still attached, therefore, nothing came over the speaker......try plugging in the headphones, then unplugging...that worked for me.