I recently replaced every cap on my Duo's logic board. Didn't take me a week, but it took me about 3 hours. Unfortunately, I didn't have this problem prior to replacement.
My question to pose to you is this-- in cases where replacing known audio-related caps hasn't fixed a sound issue, did you try dishwashing the board afterwards to remove any leaked electrolytic? It's possible the problem in some cases could be caused by the leakage itself rather than the cap that leaked. That goo is conductive and could cause small electrical shorts resulting in elusive symptoms even after cap replacement.
Read the story of my Duo here, a tale that only ended upon deep cleaning of the logic board.