This isn't entirely accurate. You can read Heidegger's Nietzsche lectures to see what he really thought of Wagner. He agreed with Nietzsche's categorisation of Wagner as a decadent, but also thought that Wagner preserved a vision of Greece and the significance of art in a low-period of German culture.
And to be fair to Wagner, Heidegger knew practically nothing about classical music and thought Carl Orff was a good composer. For him, words lent music its higher significance. So the Ode to Joy and Carmina Burana were great works of music because they supported great literary works. As a side consequence of Heidegger's critique of Wagner, any aesthetics that elevates music to an equality with poetry, or to any sphere beyond the merely diverting, is by necessity decadent and inextricably bound up with metaphysics. So the writings of Hoffmann, Schopenhauer and so many others on music become void. Heidegger particularly hated Schopenhauer.