Are the audible warning standardized?
General alerting sounds are chosen by the manufacturer of the cockpit systems. Stall warnings, autopilot disconnect alerts, fire alarms etc. are all not standardised, with the exception of the words spoken by the GPWS/TAW and TCAS system.

Whilst the exact sounds of these two systems are not standardised in the sense that a standard .wav file is to used by all manufacturers, the phraseology in their aural alerts is standard.

The Ground Proximity Warning System (GPWS)/ Terrain Awareness Warning System (TAWS) will have to issue an aural warning 'PULL UP, PULL UP' if the aircraft is approaching terrain, and 'GLIDESLOPE' when the aircraft is deviating far below the glideslope. Also the cautions ('SINKRATE.', 'TERRAIN.', 'DON'T SINK.', 'TOO LOW, GEAR.', 'TOO LOW, FLAPS.', 'TOO LOW, TERRAIN.' and 'FIVE HUNDRED.')1 are standardised. Whether it is a male, female or robotic voice is up to the manufacturer of the system, but the words are uniform across all systems. Additional cautions and aural alerts are allowed as long as they are clear and unambiguous. 

The same holds for TCAS; 'TRAFFIC, TRAFFIC.', 'CLIMB, CLIMB.', 'DESCEND, DESCEND.', 'INCREASE CLIMB,', 'INCREASE DESCENT,', 'REDUCE CLIMB,', 'REDUCE DESCENT.', 'CLIMB, CLIMB NOW.', 'MAINTAIN VERTICAL SPEED, MAINTAIN.', 'LEVEL OFF, LEVEL OFF.' (supersedes "ADJUST VERTICAL SPEED, ADJUST), 'MONITOR VERTICAL SPEED.', 'CROSSING.' and 'CLEAR OF CONFLICT' are all standard phrases which have to be implemented in this exact wording(2).



1FAA TSO C151c Table 4-1 (pages 16 -- 18)

2 RTCA DO-185B, Minimum Operational Performance Standards for
Traffic Alert and Collision Avoidance System II (TCAS II)
Version 7.1, Volume I section, 2.2.6.3: Aural annunciations)