Gold doesn't oxidise: that's why it's on connectors. Also, if you look carefully at the connector pin design on both halves, the pins self-wipe on insertion on removal.
For there to be un-removable muck on the pins not removed by the self-wiping action, it is highly unlikely an alcohol clean will do it.
And no, I was not implying that you meant people would do something as daft as this - but they do (I've seen the results) so I am inclined not to even suggest this sort of idea any more.
Invariably, bad sound comes from a bad contact. It's (as Tony suggests) either in the audio path between the sled connection and the speakers, or - more likely - bad crimping and no stress relief in the harness behind the plug in the sled.
Sorry - there's no other option: sled out, examine the connections to the plug. I am prepared to put money on the shield braid for the bad speaker channel being loose in the pin due to a bad crimp.