As Martoch said, there isn't a "magic speed" for burning audio CD's; it all depends on the media being used. I've had some generic CD-R media that consistently produced audible errors at 12x but was fine at 24X and some that resulted in coasters at anything higher than 12x. Try 24x at first, test with Nero's
CDSpeed or some other CD-R testing utility, and if you get errors, try a lower speed. If lower speeds produce errors, try a higher speed, etc., until you find a good speed for the media you are using.
Generally, higher burn speeds will produce more jitter and BLER (BLock Error Rate) errors but this is not always the case. You should be able to burn cheaper, generic CD-R's at faster speeds if the material isn't critical; you may or may not get errors. It's also possible to burn at "too slow" a speed with some drive/media combinations. Modern CD-RW drives are designed for higher speeds and may actually produce more jitter errors at slower speeds, due to being optimized for high speeds. I use
HHB Professional CD-R's for all my critical audio CD's and their 24 Karat Gold CD-R's are only available at 1x-8x, while the Silver only come in 1x-24x. Many, if not most, standalone burners for Professional audio only support up to 24x max.
Also, there isn't a "global" burn speed setting in Windows. If you set the burn speed for WMP9, you'll have to set it individually for your other burning applications as well.