I'd like to share for posterity that I literally just troubleshot a similar problem. My situation is a little different, though: I have an ESS1868f that I normally just use for MIDI (going out to a DreamBlaster over gameport), and an SB16 that is only used for OPL.

I was having an issue when playing a game with PCM audio, since of course the SB16 is plagued by the DMA clicking bug (this means that any sampled audio played through it has a noticeable clicking artifact): in order to get around it, I thought I would try using the ESS for PCM instead, but I noticed right away that the output was quite disturbed (especially when the MIDI events were being fired).

After some faffing, I found out that the culprit was the OPL itself on the ESS, even while it wasn't being used: turning the volume all the way down only for the FM portion and setting the OPL address to F0 via Unisound quieted it right down., and now I have pristine sound across the board.

When I first started recapping old electronics, I was using ultra low esr caps, the lowest you could buy. These tend to make zero difference in most digital electronics but there were two places where I did notice a difference. The biggest difference was in audio circuits. Not sure if that would help in the case of sound cards, just wanted to mention it. Ultra low esr caps shouldn’t be used everywhere but they do have uses.

midicollector wrote on 2024-03-05, 06:25:

When I first started recapping old electronics, I was using ultra low esr caps, the lowest you could buy. These tend to make zero difference in most digital electronics but there were two places where I did notice a difference. The biggest difference was in audio circuits. Not sure if that would help in the case of sound cards, just wanted to mention it. Ultra low esr caps shouldn’t be used everywhere but they do have uses.

I have been wondering about that: what is your process? Do you just shotgun all the capacitors with same value low-ESR ones?

I've also read about adding blocking caps to the power source of the ICs on the board, any luck with that? This post looked interesting, although some of it went over my head: Re: Ways to reduce the Noise of ISA Soundcards?

Yeah generally I just replace all caps. Never tried adding anything to psus, and I’ve never recapped a sound card specifically so I can’t actually say if it would help or not.