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Hi there, sorry to hear you're having trouble with the tv
there is not a fuse or reset related to the audio functions of this set. You most likely have a failed amplifier. This unfortunately is not an easy fix, as it would require replacing the entire main board, where the audio functions reside. That part is hard to find for this set, and the cost of having it serviced is going to be more than the tv is worth.
Your best bet at this point, other than replacing the tv entirely, would be to use an external sound system. You could go with anything from a basic soundbar that can sit on the stand below the tv and cost around $100, on up to a full scale surround sound system with an a/v receiver and speakers all around the room.
The benefit of investing in a sound system, instead of investing in fixing the tv, is that the sound system can be used with your next tv as well, which is important because this set is already past it's expected lifespan, and will need to be replaced in the near future anyway.
Here is an example of a soundbar that would work well with your system, and is under $100:
VIZIO S3820w-C0 38-Inch 2.0 Home The... (click for link)
It can connect to the tv, or directly to your dvr, dvd, etc