Word clock frequencies- why??

not all music is recorded at the same sampling rate.
CD's are 44.1 kHz (44100 sample per second).
DVD-Video discs are either 48 kHz or 96 kHz.
DVD-Audio discs can be sampled at many values between 44.1 kHz and 192 kHz (44.1, 48. 88.2., 96, 176.4, or 192).

if you are using an external clock, you want the clock frequency to be the same as the music that is playing back.

well, SACD's are an entirely different ballgame. SACD using a digital encoded called DSD (direct-stream-digital), while CD's, DVD's and most other digital audio file are encoded with PCM (pulse-code-modulation). CD's are 16-bit and DVD's are 24-bit (how much information is contained in each individual sample), but it's all PCM encoding. SACD's, on the other hand, have only 1-bit samples, but the sampling frequency is 2.8224 MHz (that's 2,822,400 samples per second). obviously, that's significantly higher than even the 192 kHz DVD's. but because it's an entirely different digital format, there are very few DACs that can decode DSD, and there certainly isn't a standard that's common (like S/PDIF for PCM data) for transferring the signal digitally. For these reason's, whatever SACD player you have, you most likely have to use the analog outs and let the player do the D/A conversion, and you probably can't use an external digital clock anyway.

what player do you have, and what exactly are you trying to do with the external clock?