Complex sentence: Embouchure is controlled by way of the instrument's mouths, not the player's mouth such that the player can sing-along with the hydraulophone (i.e. a player can sing and play the instrument at the same time).
Split into simpler sentences: Embouchure is controlled by way of the instrument's mouths, not the player's mouth. The player's mouth is thus left free to sing-along with the hydraulophone (i.e. a player can sing and play the instrument at the same time).