Those Voice Modifications are from the Halloween Spells in TF2. [A wikia article is here if you'd like to read more][1], but I'll sum it up for you.

There are 9 class specific spells that alter your voice, one for each class, and 1 all class spell (Voices from Below). The 1 all class spell isn't a standalone spell, but the byproduct when a class specific spell is applied to an all class cosmetic. These spells are unobtainable for now (only during Halloween) but you can purchase items that have those effects on the market still.

- Halloween Spell: Scout's Spectral Snarl
- Halloween Spell: Soldier's Booming Bark
- Halloween Spell: Pyro's Muffled Moan
- Halloween Spell: Demoman's Cadaverous Croak
- Halloween Spell: Heavy's Bottomless Bass
- Halloween Spell: Engineer's Gravelly Growl
- Halloween Spell: Medic's Blood-curdling Bellow
- Halloween Spell: Sniper's Deep Downunder Drawl
- Halloween Spell: Spy's Creepy Croon

So while the Voodoo Cursed Souls do not change your voice to low by default, a Halloween Spell such as the one listed above can modify the cosmetic so that it does lower your voice (on Halloween servers)




  [1]: https://wiki.teamfortress.com/wiki/Halloween_Spells