Geneered organisms whose entire raison d'etre is to make music.
Not just music that baselines or near-baselines can appreciate but a sort of broadband magic that sounds equally pleasant to all clades, sounding different to each type. For example a baseline won't hear much beyond the bassline and fractal percussion (the most important part of a biophonic dub symphony). But a vec or a neodolphin etc, all hear something else.
Of course it is possible for a sophont to mod themselves specifically to cycle through a million different 'frequency response' templates. But they can never understand the complexities of each harmonic like a Biophone can.
The NoCoZo possesses huge numbers of biophones in their enormous mallbitals.