There are two kinds of sounds, "celesta" for transactions and trades (e.g. http://www.listentobitcoin.com/sounds/celesta/c003.ogg http://www.listentobitcoin.com

There are two kinds of sounds, "celesta" for transactions and trades (e.g. http://www.listentobitcoin.com/sounds/celesta/c003.ogg http://www.listentobitcoin.com/sounds/celesta/c003.ogg) and string swells for blocks (e.g. http://www.listentobitcoin.com/sounds/swells/swell2.ogg http://www.listentobitcoin.com/sounds/swells/swell2.ogg). Looking at the source, you are, however, correct, that the pitches are selected at random.