Does Thundersong cancel out a Hourglass's hidden?

Yes, Brynhir Thundersong effectively prevents an opponent from playing a hidden card for the remainder of the turn once its ability resolves.

According to FAQ #5027: "The opponent cannot reveal hidden cards this turn after you play Brynhir Thundersong. Cards from hidden are also played, so Brynhir's effect prevents revealing them."

Timing and Interactions

Because playing a card from hidden uses the chain (Rule 811.1.c.3), you can still interact with Brynhir Thundersong when it is played:

1. Opponent plays Brynhir Thundersong: The unit enters play.
2. "When you play me" trigger: Brynhir’s ability is added to the chain.
3. Window for Response (The "Hidden" Window): Because the chain is open, you can play your hidden Zhonya's Hourglass in response to Brynhir's triggered ability.
4. Resolution: Once Brynhir’s triggered ability resolves, its effect ("opponents can't play cards this turn") takes hold. At that point, you are barred from playing further cards from hidden for the rest of the turn (FAQ #7748).

Summary: Brynhir Thundersong does not remove or destroy your hidden card; it simply restricts your ability to play it after her ability has resolved. You must respond to the trigger of Brynhir entering play if you wish to use your hidden card on that turn.