I happened to have 4 of the SMD 47uF caps so I replaced several of them. The sound problem was caused by either C2, C3 or both. It's probably about time I replace them all, though, since they're leaking all over the place! C2 and C3 appeared to NOT have leaked, though, so I guess it's good to just replace them all at once.

I replaced with identical electrolytics except with higher voltage rating.