Ah I know what you mean. I had a problem in the past with my sound card drivers such that not all sounds played in Oblivion, especially when after talking to an NPC they were supposed to say their goodbye lines. What happened was that the voice file for the line got clipped and it didn't play, so that the NPC that was supposed to lip-sync with the voice went by so fast, then carried on. However, talking to them directly was working fine.
With regard to the solution viable for OB, FO3, and FNV, I prematurely rest my case for lack of sufficient understanding on the workings of the game for the moment. Still, I haven't encountered dead NPCs as a direct result of having SlowMo enabled, although if you would know any other symptoms that may manifest problems with it then I'd watch out for them.