It wasn't too complex!
Each row has a short unique sound associated with it (some have a percussive sound like a kick drum and some have a musical note or chord). Each time there is a new generation, the game scans a particular column and triggers one of the sounds for each row that has a live cell in it. Each generation, the column being scanned moves to the right.
That's difficult to explain in text! It might help to imagine a vertical line moving right that plays a note every time it hits a live cell.