Depending on the concentration of defects and the length scale of the system, theory suggests that it may be possible to observe second sound at much higher temperatures beyond 120 K.
Long wavelength phonons are the familiar sound waves. Smaller wavelength phonons have different group velocities due to dispersion.
The speed of second sound is not the same speed of first sound. The speed of second sound is essentially determined by a weighted contribution of the different of speeds of the different phonon modes that collectively participate to produce a temperature wave.
Laser interference is not required. The original second sound measurements in superfluid Helium used a geometry similar to the one you describe, where a heat source (as a pulse or periodic in time) is positioned at one end and the temperature is recorded at some point(s) along the sample.
The experiment describes an unusual process observed in a system driven out of equilibrium (i.e., non-equilibrium), where, strictly speaking, the 2nd law does not apply.
