An avi file with two audio files.

Pick the one you want and select 'Delete' for the other one. (This won't damage the original file.) Then if you want to save it as a new file with just the one track, use 'save as' and 'Direct stream copy' for the audio and video and specify a new filename. Only takes a few minutes.

Simple editing of the video and you can save them the same way. If you filter or resize or make other major changes, you will have to re-encode the video, but you can still use Direct stream for the audio, as long as you don't change it's format.