Hey sorry for the slow reply - it wouldn't be the EASIEST thing to tweak, but you could save an array of voices instead of just one entry. So for example, voice = [snd_voice1, snd_voice2, snd_voice3].
Then track down the part where it plays audio, I'd just use CTRL + SHIFT + F and put "audio_play_sound", and it should take you to the function that plays it. Instead of just putting in the "voice" variable as an argument, you could do irandom_range(0,array_length_1d(voice)).
Hope that helps!