I honestly didn't know the answer and I looked for it. If I had it, I would have posted it rather than what I did.
What I was going to suggest was to make sure all 150 clips were in the order you wanted them, output the project to a video file (mpg2) , bring it back into the program and then mute the one resultant clip. Although the process is not fast, it would be thorough and more fool proof then trying to mute them one by one. You could then add the audio you want.
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BoilerPlate: To posters who ask for help -- it is nice to thank the volunteers who try to answer your questions !
Anything I post unless stated with a reference is my personal opinion.