Not that I'm an expert, but as this question doesn't seem to be attracting any answers, what are you using to connect your Nintendo to the TV (and is it safe to assume that you're talking Gamecube)?
I would have thought a scart connection would work fine: can you get a scart connector for the N64? And, more to the point, why are you still using the N64? There was only ever one good game produced for it!
try tuning in on a different channel or makew sure that the scart cable is still in there not half falling out with my dvd player it falls out sometimes so i cant get sound or try blowing in both sides of the scart soket it might be to dusty