Funny, I thought that Jazz
was silly mood music!
Gotta have the smooth Jazz-Fusion menu music.
Hehe, talk about generalising...
Any music is mood music (or at least, it can be.)
Some of it is really good, too.
Anyway, the Japanese seem very fond of their Jazz, and GT games have always been full of smooth jazz (often laid back), progressing to proper Fusion-esque pieces (say, in the tuning menu, or whatever - nice and upbeat, verging on "zany" to the uninitiated
) or even straight Funk (now we're talking...).
I don't mind it, as it's often technically impressive, and as a "musician", I take inspiration from it. But I still turn it off eventually; I don't like listening to the same thing over and over. No music when racing goes without saying.
The interesting thing is that the menu music is (arguably) becoming more "refined" from GT2 onwards - hopefully a good omen.
Brilliantly (well, I think so), the original has a great "old-skool" .mod-like vibe going on. 
Also interesting to note is the maintaining of theme from game to game, notably with the "home" music: (1), 2, 3, 4. (love that "11th chord" motif on the bass, from GT2 onwards!)
---> Actual point:
It might be nice if the game can categorise the music it uses for the different
menus (i.e. not just the races), and then pulls one to match from your own library (even better, from any "media servers" currently accessible) so that it can be less monotonous if you spend a lot of time in menus. Could make photomode, livery / track editing or tuning a bit more personal, where present.
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