How many warning signals are voiced in modern airliners?
Modern airliners have voice warnings in a number of systems like,


Ground Proximity Warning System
Terrain Collision Avoidance System
Windshear Alerts
Radio Altitude callouts
Trim
Fire Detection
Autopilot configuration
Pressurization systems
Landing gear system
Stall/overspeed warning.


The amount of voice warnings vary from aircraft to aircraft. For example, airliners.net has the following list of voice warnings in Bombardier CRJ-200 (other than GPWS and TCAS):


  "APU" APU overspeed or overtemp
  
  "Door" open or unsafe passenger door indication
  
  "Jetpipe Overheat" overheat indication in jetpipe/pylon
  
  "Smoke" smoke detected in cargo area of lavatory
  
  "Config Autopilot/Flaps/Spoilers/Trim/Brakes" Aircraft not properly 
  configured for takeoff
  
  "Wing Overheat" Overheat in fuselage wing anti-ice ducts
  
  "Anti-ice Duct" Bleed air leak in fuselage or wing anti-ice ducts
  
  "Bleed Air Duct" leak in different part of bleed air duct system
  
  "Brakes" brake overheat
  
  "Config Brakes" here again because it's for having the parking brake set 
  while airborne
  
  "Gear Disagree" disagreement between gear indications and handle position
  
  "Gear Bay Overheat" overheat in main gear bay
  
  "Nose Door" Nose gear door open after gear fully extended and locked or >250 
  knots
  
  "Engine Oil" oil pressure &lt;25 psi


Initially, most of the voice warnings were recorded voices. However, more and more modern systems (like in newer 737s) are using computer generated voices.