The most direct and usually the best way to "turn off" a signal in MSP is to multiply it by 0. However, even if a signal is multiplied by 0, MSP still works to compute that signal. So, if you need to conserve CPU usage, it's best to disable the computation of that signal once it has been silenced, then re-enable it when you want to turn it up again.
Did you know that there are several different ways to turn MSP audio on and off in Max?
As you get more involved in programming audio, it's likely that you'll want to have multiple sound possibilities available in your program, that you can switch on and off as needed. You might want to take a look at MSP Tutorial 5: Turning signals On and Off to learn about some basic ways of doing that. However, that chapter leaves out the following pretty crucial issue.