Is there an audible warning on commercial aircraft if you try to land gear up?
All turbine-powered aircraft used in scheduled airline service under CFR Part 121 are required to have an approved TAWS installed;   


  §121.354   Terrain awareness and warning system.
  
  (a) Airplanes manufactured after March 29, 2002. No person may operate
  a turbine- powered airplane unless that airplane is equipped with an
  approved terrain awareness and warning system that meets the
  requirements for Class A equipment in Technical Standard Order
  (TSO)-C151. The airplane must also include an approved terrain
  situational awareness display.
  
  (b) Airplanes manufactured on or before March 29, 2002. No person may
  operate a turbine-powered airplane after March 29, 2005, unless that
  airplane is equipped with an approved terrain awareness and warning
  system that meets the requirements for Class A equipment in Technical
  Standard Order (TSO)-C151. The airplane must also include an approved
  terrain situational awareness display.
  
  (c) Airplane Flight Manual. The Airplane Flight Manual shall contain
  appropriate procedures for—
  
  (1) The use of the terrain awareness and warning system; and
  
  (2) Proper flight crew reaction in response to the terrain awareness
  and warning system audio and visual warnings.  


The TAWS system includes the functions of the older Ground Proximity Warning System (GPWS).  

GPWS Mode 4, Flight Into Terrain with Less Than 500 Feet Terrain Clearance and Not in Landing Configuration, provides for alerts to the crew when entering a defined envelope below 500 feet (based on height and rate of descent) and the gear is not in landing configuration. There is also an alert envelope for Flaps not in landing Configuration.   

The Mode 4 alert consists of an audible alert "TOO LOW, GEAR. TOO LOW, GEAR."