Problem with volume drops with multiple tracks?

[Supershadow_X]

I'm having a problem where if there is say, a main vocal and a backing vocal and they both sing something loudish at the same time, it makes the backing track volume drop during that second or so, which makes the whole thing sound weird, like muffle-y - does anyone know why and what I can do about it?

[Supershadow_X]

I've actually solved it on my own - its because I had the tracks at different volumes. Once I'd put them all at '0' it seemed to fix it, mostly.