First open the sound in another window in QT. Do a select all or drag the selection points around to select the part of the sound you want. Do a copy (command-C), select the window with your movie and select Add under the Edit menu. That's it. Save the movie with a new name and deselect the button "allowing dependencies". Choose to save as a self-contained movie otherwise your movie only holds a reference to your original sound file. If you move that the movie will not be able to find it.
If you need to stretch or shrink the sound then the pasting part is done differently. Select the part of the movie the sound should apply to. Now under Edit select Add Scaled. This will adjust the length of the track to the length of the selection. Note that this will speed up or slow down the sound as needed. In most cases you would only do this to make slight adjustments.
I recommend practicing with short segments of video and sound first.
Comments
If you need to stretch or shrink the sound then the pasting part is done differently. Select the part of the movie the sound should apply to. Now under Edit select Add Scaled. This will adjust the length of the track to the length of the selection. Note that this will speed up or slow down the sound as needed. In most cases you would only do this to make slight adjustments.
I recommend practicing with short segments of video and sound first.