Where in the body? you need to be a little more specific for us to tell you anything other than replace all of the o-rings (which is what I would probobly end up doing anyway). If it is really hard to hear there is a chance that there is a nick somewhere on an o-ring that might not be noticeable, but that doesn't mean that that is the issue, it might be a factor of the pressure in the gun or tank being too high or another internal part that needs to be replaced. Please try to be as specific as possible when you say where you can hear the leak from.
if you can't determine where the leak's origin is fill a spray bottle with water and gas up the gun and spray water around where you think it is, it should begin to spit up water or bubble where the leak is coming out from.
About the LED, once an LED burns out it becomes a series resistor (not sure if that is the right terminology) which means that the current and voltage is going to be going straight through the burned out LED without actually giving any of it to the actual light part of the LED. In other words, you have yourself a fancy looking piece of plain old wire. Your going to need to replace it.