Stop and think about it for a minute - if the output of the DAC is good, then the output of the YM3812 must be good too. It's a digital chip, which has fairly complex programming requirements, so if it was faulty it would most likely not work at all.
The YM3812 feeds into some sort of external DAC, and off to the signal processing side of things. Now, I'm not sure what you mean about the channels being mixed together, because as far as I can tell the YM3812 is resolutely mono. I could be utterly wrong, though.
Anyway, the DAC is usually fed into some sort of buffer, and then if there are more than one output from the same DAC they're fed into a demultiplexer. Think of this as being a rotating switch, like the distributor in a car engine - it switches from one output to the next in quick succession. Try feeding a low-level signal into the signal path starting at the power amp and working back, until you find where it gets distorted.
Incidentally, 4051 and 4052 multiplexers seem to fail more often than they work.
Gordon