Slapping the unit causing the missing sound to come back tells me that there is something loose in the receiver. This could be a bad solder joint or dirty switch contact. Vibrations, such as sound, could be jarring the bad connection and causing the sound to cut out when it was just working a few seconds before. I don't believe you have a bad part in the receiver. When a solid state device fails, its gone for good. Again, it soulds like a bad connection. Finding a bad solder joint can sometimes be difficult. What you need to do is take the cover off so you can see the bottom of the PC board and using a good light source and magnifying glass, look for the bad connection. You can try running the receiver with the cover off and using a plastic pen, try pushing down on the PC board and see if you can make the sound cut out. You may be able to isolate the area by where you push on the board. Look for loose wires too. While you have the receiver open, buy some electrical contact cleaner in a spray. Radio Shack used to sell this as TV Tuner cleaner. I don't think they carry it anymore though. Spray the switches and rotate them back and forth several times. See if this resolves the problem. If you do find a bad solder joint, all you have to do is resolder it with some new solder. Make sure you do this with the power OFF. You're not looking at a big bill here. It's an easy fix once you find it.
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