I don't know if this will help. I had a similar problem. I had my bluetooth headset paired but not being used when I answered the phone, but because it was paired the phone had no sound except with loud speaker . when the blue tooth ear piece was turned off or too far away to connect the sound was ok. Update (06/27/2016): You also may try re-connecting the head set by un-pairing it. Then pair it up again.
This also happened to me after I spilt a little water on it. I thought I had fried it for sure. I took out the battery and sim card and placed them in a bowl of rice. This absorbed all the moisture. I let it sit for about 8 hours (overnight) and the next day I put it all together again started it up and I haven't experienced the issue since. I don't know if it will work for you as I don't know all the circumstances but it can't hurt I guess.