I use a combination of volume envelopes (aka "automation") and compression all of the time. For this purpose, either could work, and a combination of both might be even better. If you've never used a compressor before, for this task (lowering audience), I'd set the threshold near the peaks of the music, and use a high ratio -- that will leave the music dynamics mostly untouched and have the compression kick in when the crowd comes in. With respect to envelopes, try to change the levels as the change in clapping comes in/out. For example, start your envelope right before the crowd starts clapping, and end your envelope right as the crowd's clapping starts to peak. Then, do the opposite on the way out, start the envelope right before the crowd starts to mellow with the clapping, and end the envelope right after the crowd is done clapping. I've found that to be the least noticeable technique, but YMMV. This way you are only changing the volume during the times when the volume is already changing (clapping coming in or going out), and you're NOT changing the volume when the volume is constant (during the period where the volume of the clapping is constant) -- so the envelope ramps down as the clapping comes in, stays flat (and lower) during the time the clapping continues, and then ramps back up during the clapping trailing off. Also, try using different types of fades (fast, slow, smooth, linear) to see which one sounds the least obvious. At least that's my two cents...