First, what kind of AV cable are you currently using with your PS2?
Second, neither of the OSSC’s 3.5mm jacks can accept audio for AV1. The jack next to AV3 is audio input for AV3 only; and the jack next to the HDMI output serves as either input for AV2 or as a breakout for audio received through AV1 (togglable with a switch).
If you’re looking at getting a SCART cable, I would hold off on the digital audio stuff until you’re sure you need it; chances are, if you buy a decent SCART cable, like those from Retro-Access or Retro Gaming Cables, simply having a properly-shielded cable will eliminate your audio buzz problem. If it doesn’t, and you really want to stick with RGB over SCART, then I would recommend simply routing the digital audio around the OSSC–either run TOSLINK directly into a compatible home theater receiver, or use a TOSLINK to RCA or 3.5mm adapter to connect it to your TV.
Personally, I would forego RGB in favor of YPbPr, because it gets me progressive scan support without the sync-on-Green BS, and there’s no real difference in picture quality with a decent cable (like the official Sony ones).