Sensing signal from computer > triggering a MCU
There s a simple solution. The duration of the alarm tone may be the trigger you need for arduino. Match electrically arduino input with speaker output. A simple RC circuit may do. Add another capacitor, to integrate the sound voltage, from gnd to the input pin. This will at least soften the audio waveform. In software, make a permissive routine, that for certain amount of time must receive certain amount of pulses or analog readings. This model will ignore clicks and sudden noises.