Sounds good on paper. Until you realize that sound pressure level will drop with the square of the distance from the sound source. Even within the confines of a cave (which will channel the sound and prevent the energy from dissipating in space), there will be natural attenuation that will quickly diminish the effectiveness of any acoustic weapon. And that's discounting any artificial attenuation created by the enemy (such as doors and baffling).
Furthermore, this device requires the focused interaction of two sonic "beams": it is the harmonics generated at the intersection of these two beams that creates the disabling acoustic energy. The only way these two beams can be effectively focused is in free space (i.e., open air); you can just shoot them both down a tunnel and expect them to come together down in the terrorist's living room.
A far better tool to have in the warriors "toolbox" for clearing out tunnels and underground bunkers is the subterranean-detonating nuclear bunker-buster bomb. But the Liberals have quashed that one because it isn't happy-happy, friendly-friendly enough.