I have a movie on 815 Mb, 1 hour 20 min long, which is gonna get a new audio track all the way in WMM

But since Windows Movie Maker are cutting up inputs into mini-bits, that brings me about 150 small audio tracks in a row ..that i must add silence to manually before i can save out a test with a new music track..cause the sourch sound gotta shut up!
i dont know if its possible to gain control over the whole audio as a unbreakable single track?

its gotta be some methods