I finally found this page, which explains why you can't have if statements in the poorly-isolated subset of the language that amounts to static audio circuits, but you can have things that are morally equivalent to if statements (albeit arguably with different cost semantics) via Select.ar and Select.kr. I really don't know why they don't just compile the syntax of if statements into that and be done with it. OH I KNOW WHY IT'S PROBABLY BECAUSE THE LANGUAGE DOESN'T HAVE if STATEMENTS AT ALL and instead some OO-fetishizing message-passing monstrosity that isn't first-class enough for a compiler to conscionably deal with as a special case. Grrr.
Anyway. Here's some music that doesn't sound all that great, because I spent all my patience on just getting two different instruments going on at once.