Capacitors can and do go bad and what you describe definitely sounds like they're at fault.

Thanks for confirming Tiido! Sounds like I have a project on my hands (no pun intended)

Grab your multimeter and start to diagram out the signal path. Also, what chipsets are we dealing with? ES1688 based cards and later integrate FM with the PCM DAC, so there's no accessible discrete analog path for both FM and PCM sources. Non Vibra, non AWE64 cards generally have a discrete mixer chip that takes in both PCM and FM analog audio, so there's more room for repairable faults.

We're talking an ES688 + Yahama OPL3, and a Vibra based card.

Hmm... I don't like the sounds of that. The Vibra chips integrate bus controller, DSP, codec, and mixer. I don't see an option for the PCM signal path being broken external to the CT250x chip. ES688 I don't know as well, but I suspect it might be the same. However, because it has a datasheet, you can look at all the control pins and see if there's a possibility for channel muting caused by one of those.

As a precautionary measure, it won't hurt to check the voltage rails on the card, but it's best to do so with an oscilloscope in case there's an issue with ripple.

This conversation is drifting outside my level of understanding but that's OK, if a capacitor refresh doesn't fix 'em then neither are classic cards and can go to hardware heaven. The Vibra is defs an odd one because the software mixer knows there's an issue and insists on setting the balance all the way to the left. I've tried editing the associated .ini and it will then display 50%, but saving the config sets it back to 0.

Problems usually come with electrolytic capacitor. The other may be fine.
But I've never seen this happen on a sound card. Most of the case it was with bigger power supply caps.

Old and partially dried up capacitors can also be the culprits of an increased noise floor.

At the very least, make sure 5V is really 5V, etc.

Yes, check the voltage regulators on the card as well and see whether they output the correct values, this can make the volume drift massively or make the chips at all inoperable.