Re: 24p - 23.976 fps video & 24 fps audio
I guess it's down to simplifying it. Say you shot at 24 and then had to display on a screen only capable of 60i, as is the case with lots of older and low-end displays. The camera would either have to pull-down, or you'd have to put it in to an edit suite, pull-down and then export.
23.98 means it's close enough to 24P to pull-up if required, but for TV where it has to be compatible with old NTSC standards, 23.98 means you can deliver for HD and SD without having to do any conversions, and if everything is 23.98, it avoids confusion over what has been pulled-down and what has to be pulled-down.
It would be nice if there was a clean start with HD running at 24 or 48, but then maintaining compatibility with SD systems unfortunately stops that. They could have done it with DVB and ATSC too, but that had to maintain compatibility with analogue sets.
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