My phone does it, many other phones do it. So the problem is in android itself.
Connect your bluetooth device. Use it. Then disconnect.
Guess where your volume is set? Somewhere so loud that it sounds like you're on speaker phone - through the normal ear speaker!
People say that if you turn the volume all the way up and then down again it is fixed. Or reboot.
I say Google should fix this crap. Remember, details matter.