Right click on the track header (the bit at the left hand side of the track with the level controls etc), select Insert/Remove Envelope | Mute. If you don't get that choice then you're using Vegas Movie Studio rather than Vegas Pro - but you didn't tell us that, did you?
Only Vegas Pro has scripting. If you search for Google Scripts you'll find several freebies, but therse are also several pay-for script packages - I use both
Excalibur and
VASST UltimateS , but, as I say, you need Vegas Pro if you want to run scripts.
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