Did you clean the board before replacing the capacitors? Replacing the capacitors w/o cleaning just stops the bleeding; the next-to-invisible electrolytic gunk that leaked from the old caps is still present on the board, usually shorting things in the process.
Also, there are other capacitors in the audio circuit than the ones you just replaced. See Charlie's schematic for specifics. In addition, there is one way out in the middle of nowhere that isn't obviously connected to the audio, but it is. IIRC it's the one in bottom right corner of the above image, I think it's C512 although it could be C521 or C525. It's been so long I just don't remember, but it seems to me it didn't have anything else near it.
My Duo started in with the "scratchy" sound at one point, and I replaced everything obviously tied in with the audio. Didn't fix it. I started replacing caps one at a time, by the time I'd replaced nearly 75% of all the caps on the board, I replaced that C512 (or whatever it was) and the problem went away. This was almost three years ago, and I haven't had any sound issues since. I don't see C512 anywhere on Charlie's audio schematic, so I'm not sure what role it plays in the audio circuit but it obviously does SOMETHING.
But, again, replacing caps without cleaning the PCB (preferably via a dishwasher cycle) is.... well, think of the Duo like an engine with a bad oil leak. So you fix the oil leak, but don't replace the oil that leaked out prior to repair. The engine still has a hard time because it's missing three quarts of oil. You need to address the residual effect of the problem, not just the problem itself.
If you've already cleaned the board, good on you.... You have another bad capacitor somewhere. Just replace everything mentioned in the schematic and go from there.
