I had the same problem you have. It turns out that the speaker is in the lower cover. Not the upper cover mentioned elsewhere. Here's a picture to help me explain: http://imgur.com/akIB9Cy The upper piece in the photo contains the speaker. You'll notice the contacts on the thin portion. The lower piece is the ribbon cable that goes up the left side of the battery. Notice the contacts on the piece that hangs off of the cable. Apparently a lot of us end up compressing those contacts in the process of removing the cover that contains the speaker. What you need to do is use some tool to carefully uncompress the contacts on the ribbon cable. That should let them make contact with the speaker contacts and fix your sound problem.
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