You can also use this list to find other attributes:
http://www.tf2tools.net/utilities/schema-attributes
In that list you'll see two attributes related to voices; 1006 (SPELL: Halloween voice modulation) and 2048 (voice pitch scale)
The attribute 'SPELL: Halloween voice modulation' gives you the deep sounding voice. I believe the values for that are 1.0 or 0.0 for either on or off.
The attribute 'voice pitch scale' changes the pitch of your voice where it can sound more feminine or more masculine. A value of 1.0 is normal. Values lower than 1.0 (such as 0.9, 0.8, 0.7, etc.) make the pitch higher. Values higher than 1.0 (such as 1.1, 1.2, 1.3, etc.) make the pitch lower. Feel free to experiment.