I tried using different swf-files with different speed on different levels, but it appears that the main-movie sets the speed for them all…so no luck that way.
If you want to FAKE the effect, you can skip the frame at a tme, play frame 1, 3, 5, 7,… So that it will be looking like 24/12… Using the gotoAndPlay() function properly would help you alot… =)