Your mic is probably beginning to fail. The same thing happened to my first Evo LTE that I purchased. The entire process took about 2-3 months. Eventually, it might stop working completely.
While I'm not saying that this is definitely the case for you, it is very likely, as I had the same experience. Reflashing your firmware should be the first step, then run a RUU if the firmware flash doesn't help. If not, you might need a new phone, buddy.
My main advice is to activate Sprint TEP insurance from now ($5 per month), if you dint have any. Depending on the problem, they replace your device for free. But you can't enter Sprint with the problem.