I agree with Arachne. I find that there are few exceptions to the burn Audio CD at 16x and burn Data CD at 24-32x rule of thumb (but exceptions do exist).
Burning at 48x (and higher) can result in bad burns even with very good media or very good burners, unless the media and burner is very well matched. Burning at 40x will generally result in usable results but 32x is safer.
Burning at 24x (when supported) is often the best compromise between quality and speed.
Burning at 16x is usually the fastest CLV speed (except for a few drives) and usually provides the best chance of lowest possible Jitter, which can be important for Audio CDs.
Burning slower than 16x rarely produces better results (and sometimes worse results) except when using laptop burners and/or older CD-R media rated slower than 16x.