Audio Cues

Certain events in Left 4 Dead and Left 4 Dead 2 have associated audio cues. These take the form of sound queues that play when Special Infected spawn, or sounds that the special infected make while waiting to attack.

Special Infected
Note: For those who are not quite as musical, each special infected has a different number of notes in their audio cue.
 * 2 notes: Smoker
 * 3 notes: Hunter
 * 4 notes: Spitter
 * 5 notes: Boomer
 * 6 notes: Charger
 * 8 notes: Jockey

Boss Infected
These infected do not have preset audio cues by the game, but they do have boss music and do make their own idle sounds.