Water pressure in tub has suddenly dropped and water will not come out of shower head at all. I can hear what sounds like water running up the pipe to the shower head when I have the tub faucets on, but when I pull up the diverter for the shower, no water comes out of the shower head. Fixtures are about 20 years old. Will I have to tear out the tub surround to find the problem?
It sounds like one of your valves is not working correctly. It could be that the handle is slipping or that the valve is clogged. Does it feel like the handle is turning the valve, or is it slipping? If its slipping you can probably just tighten the screw.
As far as replacing it, you might get lucky and find a replacement valve, but in the event you don't, they make remodel plates which allow you to relatively easily replace a 3 handle setup with a 1 handle faucet. The remodel plate covers up the area in the wall/tile you'd need to remove to swap it out; you don't have to tear out the entire wall. If you can get in behind the shower you can probably do it without the remodel plate.
first, i assume you have pressure everywhere else in the house, both hot and cold. The tub is working normally? Second,the sound of water in a valve can be very deceptive. a small amount of water can sound like normal flow, also air flowing back into the system can make a similar sound. If you have pressure in the lower tub faucet, then it has to be the valve or a major leak between the valve and the shower head. A bad valve would rarely require replacing the valve housing, and most repairs can be done by without going into the wall.
How old is the valve? single or hot and cold? brand? Most valve stem assemblies can be repaired or replaced by turning off the main water supply and unscrewing the valve handle or cover. This is not a big job, but often requires some finesse to do it properly on older systems. Newer designs are easier to remove, but harder to put back together properly. A lot of step by step instruction are available on the web once you know the brand and model.