Talk to Mulush about Omluag

Walkthrough
If the Dragonborn speaks with Omluag, one of the two workers at the smelter in Markarth, he will mention that the overseer Mulush gro-Shugurz is working the men too hard. The Dragonborn can then offer to speak with the Orc.

If the Dragonborn's speech skill is sufficiently high, a simple talking-to will suffice. Otherwise, intimidation might be necessary. Sprich mit Mulush über Omluag