Learn German: Das Gebäude, das ist leider so hellhörig, wenn sie oben trampeln. - The building, it's unfortunately so poorly soundproofed when they stomp upstairs.

This is a common feature in spoken German called left dislocation (Linksverschiebung). To heavily emphasize the topic (Das Gebäude), the speaker pulls it entirely out of the main sentence and states it first. The second das is a demonstrative pronoun (meaning "that" or "that one") which picks the topic back up and acts as the actual grammatical subject of the sentence.

Doesn't the verb have to be in the second position? Why doesn't ist come right after Das Gebäude?

The strict "verb second" (V2) rule applies to the core sentence. Because Das Gebäude was pushed outside the main clause to the left, it doesn't count as the first position of the main clause. The actual main clause starts with the pronoun das (position 1), which makes the verb ist take its correct spot in position 2.

hellhörig translates to "poorly soundproofed", but what is the literal meaning?

It combines hell (bright, clear) and a suffix related to hören (to hear). Literally, it describes a space where sounds come through "clearly." It's the standard, everyday word Germans use to complain about thin apartment walls!

Who does sie refer to in the clause wenn sie oben trampeln?