Hi again Rick - hope you are having fun working your way through the tutorials!
Earlier you said:
..and how can I remove the sound that was recorded underwater? Me heavy breathing !! to replace it with Audio sound of the waves
To adjust the volume on all the clips in your project go to the Menu - Edit>Select All then click on the little gear icon on the bottom left of the clip, select Audio Adjustments, then in the pop up reduce the audio volume slider to zero or whatever level you prefer. Or if you just want to reduce the volume on one clip just select (click on) that clip and follow the same procedure.
Sounds like (no pun intended!) you've figured out how to add audio tracks. Adding the track a number of times is fine, providing you "join" them up carefully so there are no blank gaps. Click on the audio track (underneath the video) then click on the little icon and select "Clip Trimmer". This will reveal a window containing the audio track and showing the wave forms. You can easily drag the ends to get rid of blank areas at the beginning and end of the clip.
After trimming each audio clip, it's then easy to drag each track to effectively "join" on to the others. Alternatively, if you don't want to go through this process, simply drag tracks over each other to "mix' tracks. Each track will assume its own position under the video. It's easiest to do all this if you adjust the number of frames per thumbnail to give you an extended view of the clips. Just drag the slider at the bottom right to show half a second or incrementally up to 30 seconds for each clip. There is also a setting to show All, giving you a single frame thumbnail of the clip.
iMovie also provides you with an audio "ducking" feature, which can be very useful when manipulating audio levels for individual tracks. But I won't go into this here - best to read up on it first and/or watch the tutorials!
Best of luck with all this Rick - enjoy iMovie '09.
John