When Doom casts Scorched Earth and then enters the fog of war, the Scorched Earth sound continues playing at his last known location. It lasts until the spell expires, or resumes following when gaining vision over Doom again.

Similarly, when he casts it while you had no vision over him, Scorched Earth's sound won't play at all even when you gain vision over him.

The sound should fade in upon gaining vision, and fade out upon losing vision over the caster.