You can see (and I imagine adjust) the pitch changes in descr_sounds_units_confirm.txt between general, heavy, medium, and light. I wonder if these can be tweaked for a "sucked helium" effect. The sound files listed are for group selections though, maybe these are just defaults.

export_descr_sound_units_voice.txt looks like it points to the various wav files for unit action replies (packed in the sounds/idx files).