A type of Special Attack. From the Accelerant core rules book:

"Someone calls out "By My Voice" and an effect and everyone who hears it will be affected by it. You cannot defend against Voice attacks by intentionally obscuring the sound of the verbal with loud noise or by plugging your ears."

It is important that players or NPCs who are giving beneficial effects to allied characters be as loud and clear as possible - if the other person could not hear or understand what was said, they are unlikely to acknowledge receipt of the beneficial effect. This is particularly true in situations where there are a lot of characters or NPCs making noise near by due to large scale combat or poor acoustics.