Deepened/Slowed Voice With Voodoo-Cursed Soul?

Often during Halloween events, one of the funniest things in the game for me occurs in the form of heavies and scouts and the like obtaining deepened/slowed-down voice commands and death/pain sounds.

I was sure it was because of the cursed voodoo souls, so I went and bought a few, but I can't seem to figure out the voice.

Surprisingly the only reference I can find that even mentions the voice modification is on the wiki: "The souls also affect the classes' voices, making them sound slower and deeper, and give the Scout and Soldier special voice lines." But I hear no difference from my lines, and most of the time when following another soul player (whether or not I have Halloween items on) I still hear no difference. Does it need to be Haunted quality or something?