SaxxonPike wrote:

But some of them have a 24.576mhz crystal, and some of them don't.

Which cards don't have? If you post a link to a good high-res picture, the reason might be obvious.

SaxxonPike wrote:

The datasheet for the OPL3-SA3 for example doesn't say anything about these clocks besides the information given in the pinouts for each package.

Which datasheet you are referring to? YMF715 datasheet does say a lot about it, like both frequencies are required.

Phil's Computer Lab features the Audician 32 Plus, which is missing it:

http://www.philscomputerlab.com/audician-32-plus.html

However, my card is pretty much exactly the same, but with the crystal:

OK, I was only able to find YMF-715 datasheet but not YMF-719, they might be different in this regard.

In the YMF-715 datasheets there is an example how to use a PLL chip to generate both frequencies from a single crystal with a PLL chip.
But there is no separate PLL chip on the photo with single crystal, so that's not it.

So, the 24.576MHz is really missing from the board. Perhaps the YMF-719 has an internal PLL, but I don't know.

If that frequency is not absolutely necessary on YMF-719, maybe the drivers know from PNPID or otherwise detect it does not have the crystal so it does not use rates based on that clock.

But other than the info above, I don't have an explanation.