mux problems

[hammy1]

Hey, I'd like to sync seperate video and audio that are in different formats together, but i dont know where to start. Is it possible and how would i go about it? I've been downloading and converting flv. videos off of youtube but on this perticular video i'm only able to get audio and video seperately. Alternatively if someone knows a good free flv. converter it would probaly save me alot of hassle.

Thanks in advance,
Dave