About 500 QI buzzers

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Those unfamiliar with the show may note that the testing of the buzzers typically occurs in groups of four, culminating with Alan Davies.

These observations from the Wikipedia entry:

Each panellist has a buzzer, with the sounds of all four often being based on a theme. They are demonstrated at the beginning of the programme, but are sometimes changed in some way for repeated use. Davies' buzzer usually subverts the theme established by the preceding three. Comical twists include in the ninth episode of series B (Bats), when all the first 3 buzzers were bells, then Alan's buzzer turned out to be a male voice (Leslie Phillips) saying "Well hello! Ding dong!" ...

In episode 5 of Series A, rather than a comical buzzer, Davies set off the forfeit alarm, (suggesting he sets one every time he offers an answer) meaning he started the show on -10 points before a question was asked (it was later changed to the sound of a duck quacking)...

Sometimes questions are based on the buzzers themselves, usually Davies'. For example, one of his buzzer noises the Series D episode "Descendants" sounded like a Clanger, and the panel had to try and guess what was being said (the answer being "Oh sod it, the bloody thing's stuck again.") In the Series F episode "Fakes and Frauds," all the buzzers sounded like ordinary household objects, but then turned out to be the sound of the superb lyrebird mimicking the noises. Davies's however, was again an exception; his buzzer, which sounded like a telephone, really was a telephone and not a lyrebird mimicking one.