That's because of the 50/60hz timing issue. All NTSC games run at 60hz, or about 17.5 percent faster that their PAL counterparts. Subsiquently, any video that is supposed to be syncronised to an audio track is too slow, hence the sync problems.
To fix this, you need to perform the 50/60hz mod on your MD. Details are outlined in the Miscellaneous section. There is also a topic running in the Genesis section of this forum with the same topic.