The gas near the center of the Perseus cluster exhibits quasiperiodic pressure oscillations that can be interpreted as sound. I found a paper that says that these pressure waves have a wavelength of about 11 kiloparsecs, a speed of 1170 km/s, and period of 9.6 million years. (My math says that 11 kps / (1170 km/s) equals 9.2 million years, not 9.6 million - but whatever, close enough.) This corresponds to an infrasonic (to say the least) frequency of about $3 \times 10^{-15}$ Hz.
But what's the amplitude of these pressure waves? The paper only seems to discuss relative amplitudes at different locations within the cluster, not absolute amplitudes.
