A Klaxon is a audio signal that is activated when a planet, ship, station goes on any "Alert" status, especially on "Red Alert". The klaxon alert devices have varying tones from ship to ship, installation to installation. These have been heard on the Enterprise, DS-9, Voyager repeatedly to indicate everything from a "General Quarters" alert to a "Red Alert/Battle Stations" alert.

Shows that used the Klaxon:

• Star Trek: TOS
• Enterprise
• Star Trek: TAS
• Star Trek: TNG
• Star Trek: DS-9
• Star Trek: Voyager
• The Star Trek Movies

Klaxon article at Memory Alpha, the wiki for canon Star Trek.