Voice of Heresy is a bundle of Warhammer 40K audio books. All these are set during The Horus Heresy, 10k years before the current W40k setting, about Horus' attempt to claim the Imperial crown. $18 for 23 books is pretty good though some are short stories.

The Horus Heresy

#1 Horus Rising (12 hours)
#2 False Gods (11 hours)
#3 Galaxy in Flames (9 hours)
#4 The Flight of the Eisenstein (12 hours)
#5 Fulgrim (15 hours)
#6 Descent of Angels (10-1/2 hours)
#51 Slaves to Darkness (11 hours)
#52 Heralds of the Siege (11-1/2 hours)
#53 Titandeath (13-1/2 hours)
#54 The Buried Dagger (12-1/2 hours)

Horus Heresy: The Primarchs

#1 Roboute Gulliman: Lord of Ultramar (5 hours)
#2 Leman Russ: The Great Wolf (6 hours)
#3 Magnus the Red: Master of Prospero (6 hours)

Audio Dramas - short stories later incorporated into anthologies

The Binary Succession (part of #48, 75 minutes)
Garro: Burden of Duty (part of #42, 35 minutes)
Garro: Sword of Truth (part of #42, 2-1/2 hours)
The Long Night (part of #35, 42 minutes)
Dark Compliance (part of #52, 1 hour)
Garro: Legion of One (part of #42, 1 hour)
The Eagle's Talon (part of #35, 39 minutes)
Garro: Oath of Moment (part of #42, 71 minutes)
Iron Corpses (part of #35, 40 minutes)

So it turns out the bundle is 10 novels, 3 half-novels, and 9 short stories that are already part of anthologies in the main Horus Heresy line. Still worth #18 though I wish Humble Bundle (and Bundle of Holding for that matter) had better product descriptions.

Reading up a bit more this W40k setting is only for the miniatures game, there are no RPG books about this. Therefore probably won't buy.