Most ICs would actually be quite happy with +5V alone. But, since you want to decouple from all the noise in the system's power rails, often linear regulators are used to provide a clean(er) 5 volts on audio cards. To do this, you need to come in somewhat higher, which ends up being the second positive voltage (+12V) provided on the ISA bus. So, in most cards, it's really not about the ICs needing another voltage, but simply the need and goal to have low noise in the analog part.
I'm not saying there might not be an odd-ball card with just only a 5V requirement, but that one would probably be a pretty bad beeping and squeeking noise machine if the ICs are 5v dependent 😀
But, if someone at that time managed to build a card with 3.3v ICs in the analog part, a single 5 volt supply would actually work, though. Maybe someone else knows if those existed.
Edit: Looking at pictures of the OPL2LPT it seems it's using a USB port as power supply. So the +5V seem to not be coming out of the system itself, which probably is why it's getting away with not using a regulator (I'm guessing here, though).