As far as YMF71x cards go, when there is no oscillator onboard there's a PLL that takes the possibly inaccurate 14MHz clock from ISA and generates 24 and 33MHz the chipset requires to operate. Two separate oscillators is certainly best for this chipset and a dedicated oscillator on chips that use one clock.

Some chipsets like OPTi and C-Media ISA ones are designed to work off 14.31818MHz clock, with intent that they can use the one from ISA for lowest possible cost implementations.

There could be a minuscule pitch difference if the clock is slightly off or no sound at all if there is something wrong with the ISA signal altogether.

Out of interest: what happens if you mess around with ISA bus speeds with one of these cards without their own oscillator? I know the bus itself is specced at 8-8.33MHz, with the relevant high-speed clock supposedly being 14.31818MHz. If the motherboard has a separate oscillator for the "high-speed" signal, it shouldn't matter, but there are motherboards where all clocks are derived from a single oscillator. Is cards doing completely wrong tones a thing in those cases?

The 14.31818MHz signal shouldn't change at all on ISA slot so cards should not start misbehaving.

Thanks for the excellent reply Tiido 👍