  Increasingly precise astrophysical observations of the last decade in
combination with intense theoretical studies allow for drawing a conclusion
about potential Quark Matter presence in Neutron Stars interiors. Quark Matter
may form the Neutron Star inner core or be immersed in the form of bubbles, or
droplets. We consider the second scenario and demonstrate that even a small
fraction of quark matter bubbles can lead to a high nonlinearity of the sound
wave. Below the bubble resonant frequency the sound speed is lower than the
ambient value. At the resonance it sharply grows. The peak is constrained by
viscous dissipation. Above the resonance the speed exceeds the pure neutron
star matter value. The dispersion equation for the bubbly neutron star
compressibility is derived.
