Tait Watson accounced on the official forums that many of the helmets with no voice modulation will have voice modulation starting with Patch 2.7.

/u/swtor_miner datamined the information for us and we updated all the appropriate posts (except four) with the new information. Our Voice Modulation page groups all of the helmets with voice modulation into categories with vidoes so you can hear what they sound like.

Centurion Force-Master’s Headgear
Battlemaster Force-Master’s Headgear (files not updated with new information)
Battlemaster Stalker’s Headgear (files not updated with new information)
War Hero Field Tech’s Headgear
Elite War Hero Field Tech’s Headgear
War Hero Field Medic’s Headgear
Elite War Hero Field Medic’s Headgear
Trophy Hunter’s Mask
CZ-13K Guerrilla Helm
Mask of Nihilus
Darth Malak’s Jaw Casing
Mantellian Separatist Helmet
Battleworn Triumvirate Helmet

Kell Dragon Stalker’s Headgear
Pristine Ardent Warrior’s Headgear
Pristine Ardent Blade’s Headgear
Shadowed Force Lord’s MK-1 Headgear
Shadowed Force Lord’s MK-2 Headgear
Shadowed Med-Tech’s MK-1 Headgear
Shadowed Med-Tech’s MK-2 Headgear
Shadowed Mender’s MK-1 Headgear
Shadowed Mender’s MK-2 Headgear
RD-31A Master Striker Helmet
Pristine Battlemind Headgear (files not updated with new information)
Verpine Foestopper’s MK-1 Headgear
Verpine Foestopper’s MK-2 Headgear
TD-21A Dread Scout Headgear
Kell Dragon Weaponmaster’s Headgear
Underworld Weaponmaster’s Headgear
Black Market Foestopper’s MK-1 Headgear
Black Market Foestopper’s MK-2 Headgear
Arkanian Weaponmaster’s Headgear
Kell Dragon Field Medic’s Headgear
Kell Dragon Combat Medic’s Helmet

Ancient Ardent Blade’s Headgear
Ancient Battlemind’s Headgear
Ancient Exemplar’s Headgear
Ancient Seeker’s Headgear
Bounty Tracker’s Hat
Contract Hunter’s Headgear
Core Miner’s Headgear
Dread Host Headgear
Dreadseed Helmet
Elite Regulator Faceguard (files not updated with new information)
GSI Tactical Assault Helmet MKI
Phobium Onslaught Helmet (open.faced – no modulation)
Pilot’s Helmet
Primeval Battlemind’s Headgear
Primeval Exemplar’s Headgear
Primeval Seeker’s Headgear
RD-05A Sniper Headgear
RD-05B Gunslinger Headgear
Shock-Dispersing Headgear
TD-05A Marksman Headgear
TD-05B Deadeye Headgear
TD-17A Imperator Helmet
TH-05A Operative Headgear
TH-05B Scoundrel Headgear