For ambient sounds you can have bubbling chemicals, howling wind outside, a distant piano tune, a scratchy record player, mice in the walls or the distant crackling of a fireplace. It's more about having some sort of directional noise to increase the sense of immersion than anything specific. Since you do have the puzzle part built already, you can try having it as a game mode in addition to the sandbox. Maybe puzzle mode can unlock new chemicals for you to use in the sandbox?