AnalysIR:
Should be a fun project.Step 1: Get your arduino controlling the AC unit using the IRremote or IRLib library.
Step 2: Use your PC.MAC with some free voice recognition tool to send the IR command via the Arduino
Step 3: Investigate how to get voice recognition working directly on an Arduino...this will be the most challenging bit.It should be possible to configure special keywords to minimise false recognition ("COMMAND OFF", "COMMAND ON" etc.)
As an alternative, it could be much cheaper to just set up a sound sensor and just listen for 3 hand claps or 4 hand claps. But as the previous poster suggested, 100% accuracy may be a dream particularly with a TV in the same room.
Bluetooth control via mobile phone may be a better alternative.
For Step1 check out:
Air Conditioners: Recording long Infrared Remote control signals with Arduino
Air Conditioner Record long Infrared Remote control signals - Arduino
All commands will be concluded with an OK verbal command, like Windows' speech recognition. Also, step 3 is what I want to do from the get-go, are you saying there is nothing that fits the bill?