Are you playing on PC or console, and is the option missing entirely from your menu?
If you are using an older disc version or a specific regional repack, you may need to manually move PC_English.forge localization.lang files into the main game directory. Why it Matters --- Assassins Creed Brotherhood English Language Pack
This fan-made mod upscales the original English voice files from 16-bit to 24-bit audio, reducing compression artifacts from 2010. It makes explosions, sword clangs, and whispers clearer. Are you playing on PC or console, and