The Duo lives! Another one saved from the scrapheap.
I did the cap replacement and the audio seems to sustain itself at a constant level. No scratchiness. I didn't play for long, only 5 minutes or so due to time constraints but I'll give it a good run after work tomorrow. Hopefully it won't pull the fade-in fade-out game with me after extended play. That'll be the real test.
Thanks again Necromancer for pointing out that bit set, I stopped by my local Harbor Freight after work and they had exactly one of those sets left. Unfortunately, that $4.99 sale was an exclusive web-only offer. So the guy met me halfway and sold it to me for $7.99 ($12.99 normal retail).
While I was in there I also adjusted the laser which has been giving me the singing treatment lately (no gameplay problems yet, but I hear the singing isn't a good sign). I adjusted 90 degrees counterclockwise and she purrs like a kitten again.