Here is a list of all of the available voice effects that can be applied to Sims (and Pets from my testing) that are found in the various FullBuild and DeltaBuild files. Not all of them are necessarily intended for use on Sims themselves though.

If using Sims 4 Studio, these Instance numbers can be inserted into the "Voice" text box found within the CAS Part resource to alter how a Sim sounds while wearing the edited CAS item. An example of this would be the bear suit from Outdoor Retreat that muffles the sound of a Sims' voice while wearing it.

Please note that not all CAS assets appear to support this functionality. A voice Instance can be entered but it may have no affect. Tops, Bottoms, and Full Body assets all seem to function. I'll leave it up to you to test which others do or don't.

BASE GAME
fullfacedcostume - D6CAA881E4F67C1B
ghostvox - D3582E8CD8AFF609
ghostvox - 86E8BEE0FE8202C7 (different from above)
reaper - 8844D56588FD239C
stereo_headphones -74E6339C3EC8BD9D
stereo_teenmoodswing - 6CB560FD842D0C91
stereo_quality_low - CBA78699D6FBB014
stereo_quality_medium - 91226DC98E600FFF
stereo_quality_high** - 2CED8F6862163512
stereo_volume_zero - 321083457F7579E7
stereo_volume_low - FE2211C25E1D66D1
stereo_volume_normal - 5F9C854FA6D88A94
stereo_volume_high** - CD2F7A45D71884A5
observatory - 5BAD003751D440E4
observatory - CD4B95F4C4BE467B (different from above)
sleepingpodvox - 3F52E2F18C2CB4F6
dsp_closet - 932C54CA028B5A19
weldermask - 52DC9278D8CBA0BC
micboost - 6EED391D4F765D93
micboost - 929BF082217255DB (different from above)
sylvanglade - 5CFC3A22566EB505
cave - 1CB6F57EF1043DDE
humanoid_robot - EC86EE3C20DA03F9
sleepingpodvox - 3F52E2F18C2CB4F6

GET TO WORK (EP01)
alien_flanger - 6AA94E851EA1D724
alien_filter - 6178CDD5956EDEF7
alien_vibrato - 456348D5257C1174
alienvoice* - 0D490FC5575D0A72
frontdesk_page - DF08BD8C932FE269

GET TOGETHER (EP02)
djbooth_volume_low - 51D63AE13D716509
djbooth_volume_normal - 4E1F5C57BC0C575C
djbooth_volume_high** - 602271BB5611421D

SEASONS (EP05)
flowerbunny - 53246483DB8DDA90
scarecrow - 52957BB022B6B7D2

OUTDOOR RETREAT (GP01)
tent - 8C09427E9FCCC442

* The alienvoice is not named as such in the code and is a collection of the other three alien filters applied all at once.
** The coding for these effects is either empty or set to have no effect on a Sims' voice.