Impossible timing = bad drum sim.
I like the cocnept, but there's too much silence at the start of every drum sound effect. When I click my mouse in time with the beat, each click needs to play the sound instantly. Even a fraction of a seconds' delay ruins the whole thing.
Unfortunately, I'm thinking Flash might not be fast enough for this kind of project. If you're using Flash 8, there are a few things you can do to speed things up. Start by ensuring all vector objects get cached as bitmaps, by not rotating anything in realtime, and by setting a very high frame rate.
Also, reusing the same sound object (or a finite number of them... say, a series of 8 snares that get stepped through every time a snare sound is played)) instead of creating one every time the button is clicked can help, by reducing the ammount of garbage collection going on. Hiding buttons and menus when they're not on the screen instead of creating and destroying them dynamically can help with garbage collection, too.
Good luck. This seems like a really cool idea. Without a fast, responsive implementation, though, a steady rythym is impossible, and a drum is basically useless without rythym.