This question has been answered, and could have been found using the "new" search feature this board has. However, since I have some free time, I'll give a small answer (search for a better one).
The N64 doesn't have an audio specific processor. Thus, to allow games to run faster, the music was played back at a slower samplerate, to save processing time. This is why some games have 21 khz for their sample rate. Mouser X over and out.
Try searching for "samplerate" as one word. All the results I got were pertaining to your question, almost specifically (though, the earliest ones were somewhat inderectly). Also, there's a thread still on the front of the forums, that answers your question. It came up in the search results as well. On the forum front page, try about half way down or so? Mouser X over and out.