If your 5150 is off warranty, you have a serious problem.
Replacing the motherboard in a laptop is a very tricky and potentiallydangerous thing. If you are not a trained technician, I would recommendthat you take it to a professional.
Also, the cost of the motherboard and its installation is most likelyto be close to the price of a new or used machine. Perhaps this is abetter option.
Another thought occurs to me, if the machine works fine in every other way maybe you can learn to live without sound?
More info you might find interesting is
here
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