Home on the range (selection)
Yesterday I had the task to add an alternate language commentary track to a DVD. It was a quick job that didn’t need any finessing of levels. They wanted the original soundtrack to be ducked so the original atmosphere was still audible, but quiet enough that the new dialogue was clear.
On a 90 minute movie, it takes a while to render the audio every time the level changes on a track. I used the Range Selection tool (ggg) to choose a small section of a clip. When I then rendered the audio (Command-Option-R), only the part of the timeline that was selected was rendered – quicker than rendering all the audio for the 90 minute clip.
You can also use the Range Selection tool to apply filters to part of a clip without blading it.
alex4D old blog 
Was just thinking that an easier way to save re-rendering all the audio, is simply to cut the audio with the knife tool so whats after the change in audio is not effected.