From the Citation Mustang Operating Manual found at:
http://www.smartcockpit.com/docs/Citation_Mustang-Landing_Gear_and_Brakes.pdf
"Aural Warning The warning/caution advisory system provides a landing gear aural warning if one or more gear are not locked down and either of the following situations occurs:
• One or both throttles are retarded below approximately 85% N2 and airspeed is below 130 KIAS. Pressing the HORN SILENCE–PUSH button on the gear control panel (see Figure 14-7) silences this warning.
• Flaps are extended beyond the TAKE OFF AND APPROACH setting. In this situation,the aural warning cannot be silenced with the HORN SILENCE– PUSH button."
Our Ultra has an airspeed warning at 150 knots but as time goes by the warning starts coming on at slower speeds or not at all. Over 12 years we have probably replaced 2-3 of these switches.