As the good title says, I'm looking for a sound card which only needs +5V. If no such thing, I wonder which parts require the other voltages: main chip, amplifier ,... ?
As a related example, the OPL3LPT works great off the parallel port and I doubt there's anything other than +5V there.
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).
The main point of sound cards using +12V (and sometimes -12V) is for the analog audio amplifiers. There are very cheap but sufficiently good general-purpose amplifier chips ("operational ampliefiers") that can not process signals exceeding 2V below the positive power supply or 2V above the negative power supply (e.g. the TL074). If they were powered by +5V only, they could only process signals between 2V and 3V, which is a span of just 1V, which is not enough even for line-level signals. You can choose more modern chips that get closer to the supply voltage (but they are more expensive), or you can just use +12V and possibly also -12V to supply these chips and get away with the cheapest chips. Getting +12V on an ISA card is virtually free.
Indeed, often, lower voltage analog chips are indeed powered by regulators that need a higher input voltage than their output voltage (usually +12V -> +5V) to reduce noise, as the +5V supply on the ISA slots is often quite noisy (but it wouldn't be better on a USB port powered from an ISA or PCI slot), and furthermore, most old sound cards are able to power 2W into passive speakers, which typically requires more than 5V supply voltage.