For Sirens see already sigma 280, and again sigma 282.
The present item -- identical in Photius, Lexicon sigma118 Theodoridis -- derives from the description in Plato, Republic 10.617B (web address 1 and note 2 there), of eight Sirens perched, each on a circle of the revolving vault of the heavens, and uttering a single note, blending in harmony into the music of the spheres. Compare Iamblichus, Life of Pythagoras 18.82ff.: "What is the oracle at Delphi? The tetraktys, which is harmony, in which are the Sirens." This answer is elucidated by Delatte (see bibliography below).
For the Platonic word for "note" (to/nos) the lexica use du/namis ('power'; see web address 2, LSJ II 2 e), which carries a number of overtones exploited in the mystical theories of the Pythagoreans and others concerning the call of the Sirens to the soul of the wise man to leave mortal pleasures for spiritual life (Plutarch, Quaestiones conviviales 745D-F; cf. Buffière 474-76, and References). But in this Suda entry the Sirens as notes or powers seem located inside the soul; cf. Iamblichus, Life of Pythagoras 16.68ff.