You could also check the 2 rows of smt caps and resistors behind the keyboard connector, near the audio outputs. I bought a motherboard from ebay and later another one with a known audio fault from Amibay (spares or repair). Both of them had one of the resistors cracked which was the reason for the Audio being so crackly. They weren't the same resistor in each case but they now work as they should although the resistors I used were a physically larger version as that is what I had at the time.
Mind you I don't know of anyone else who has had this problem, I may have been just unlucky/lucky with the fault
