So I know that ‘casting’ the Primarchs in a big-budget adaptation of 40k has long been a pastime, but I thought it might be interesting to see how the Black Library authors envisions the demi-gods speaking. Below is every instance I have found (thus) where an author explicitly describes the speech of one of the Primarchs in the Horus Heresy.
I may well have missed some, so feel free to add on any that are omitted. Intentionally or not I do not have any quotes from after the various traitor Primarchs achieved daemonhood.
Alpharius Omegon
"There was a quality to it that was quite distinctive…" "…deep…" -Legion
Angron
"...rough, brutal." -Galaxy in Flames
"...voice in his ear was a rasp, a rumble, words like handfuls of hot gravel." -Tales of Heresy
"…was the like fall of cities, rumbling and booming across the vast battlefield." -Vulkan Lives
"…voice was a guttural, sticky snarl." -Betrayer
Corvus Corax
"…soft, yet laden with quiet authority.” -Fulgrim
"…rumbled…" -Deliverance Lost
Ferrus Manus
"…roared…" –Fulgrim
"…voice was the steely grind of a foundry's bowels." -Shadows of Treachery
Fulgrim
"The cultured accent of Old Terra again flavoured every syllable, and Julius found himself beguiled by the timbre and cadence of the primarch’s words." "…was like music, each syllable pronounced with perfect pitch and tone as though no other sound could have filled the air so appropriately." -Fulgrim
"…lilting and golden." -Galaxy in Flames
"…words were lyrical, musical and rich." -Shadows of Treachery
Horus
" …deep rumble." " His voice was like honey, like steel, like a whisper, like all of those things mixed as one." -Horus Rising
"…powerful voice." -False Gods
"...stentorian…" -Fulgrim
Jaghatai Khan
"He spoke sparingly. When he did, his voice was cultured, tinged with an aristocratic idiom." –Scars
Konrad Curze
"His voice was a ghost's breath, all sibilance and subtlety." -Shadows of Treachery
"..rasped a death-rattle voice." -Unremembered Empire
Leman Russ
"…coarse and heavily accented. The guttural bark of his voice was like Wyrdmake’s, yet … a studied edge to it." -1000 sons
"A voice rumbled something…the voice was so deep, it sounded like an extension of the seismic turmoil beneath them." "The voice rumbled again. It was louder this time: the throat-noise of an animal mixed with a volcanic detonation, the efurious blast of the top coming off a mountain." "The voice muttered a third noise, a softer growl..." "felt [his] voice vibrate his diaphragm." " The voice trembled him again." "The distinctive wet leopard-growl of the Fenrisian astartes haunted every syllable of his words." –Prospero Burns
Lion El'Jonson
"His normally melodious voice was subdued." -Fallen Angels
"…his voice deeply musical and filled with sonorous tones that seemed to seep beneath a listener's skin and thrill the nerve endings below." -Descent of Angels
Lorgar
"…his voice a hymn…" -Shadows of Trechery
Magnus the Red
"…rich baritone laden with subtle layers of meaning."-1000 sons
"…sonorous, a little mournful, rich with the accumulated cadences of a hundered dialects." -Scars
Mortarion
"…low and firm, at odds with the ashen, hairless face" -Flight of the Eisenstein
"…his voice robust and resolute in contrast to his pallid features" "laughed, the sound like mountains collapsing." "rustle-soft tones like the dry hiss of wind over aeons old sand dunes" -1000 Sons
Perturabo
"Like glaciers grinding together…" -Angel Exterminatus
Roboute Guilliman
"…rumbled like distant thunder." -The First Heretic
Rogal Dorn
"Dorn’s voice was a low thunder…" -Flight of the Eisenstein
"…voice was low and astonishingly soft, like the lap of water on a distant beach, but there was a strength running through it, like the tension of a steel cable." -Horus Rising
"His voice had the timber of a hammer thudding into timber." "…a hoarse growl…" -Shadows of Trechery
"…as hard and unyielding as the stone of the mountains." - The Outcast Dead
Sanguinius
"…his voice surprisingly gentle. For all its apparent softness, there was violent strength concealed within it, like a riptide beneath a placid seascape." "…soft…" -1000 sons
“…his voice was, as ever, like music… pain deep within it." -Unremembered Empire
Vulkan
"…uttered a deep, abyssal voice" -Vulkan Lives
So, who would YOU cast as Mortarion for Horus Heresy: The Saturday Morning Cartoon Show?
Myself, I long for the late Tony Jay