Hi Si......in the timeline do a 'clip' 'split' at the beginning and then
again at the end of the audio portion of the video you wish to have as your
sound. Then on the music track click on the left portion.....and then
'right click' your mouse and 'fade out' the sound. On the other side to the
right....click on the right portion of your music and then when you right
click do a 'fade in'. On the middle portion which you split apart from the
longer movie you should click on the music track (small part you separated)
and then right click on it and choose either 'mute' or the 'volume' which you
can slide all the way to the left (no sound). If you want the sound on your
main video louder, right click on the clip you separated and choose 'volume'
again and you can slide the slider to the right to make it louder. Make
sure you 'preview' this so that the sound is appropriate to what you want.
Good luck.