Here h(x, y, t) plays the role of exterior mass forces, Φx and Φy are components of the gas flow velocity vector in the respective directions, and Φ(x, y, t) is the potential of the velocity field. We assume that the speed of sound is zero. Equation (1) is a quasilinear hyperbolic equation independently of the sign of Φx. It is required to study the Cauchy problem and initial boundary-value problems for Eq. (1) in different domains. These problems are rather difficult because of certain specific characteristics of (1), and this fact explains that the paper [2] and the monograph [3], where some attempts to study initial boundary-value problems for (1) were made, are the only references on the subject known to the author. A drawback of the main result [3, Chap. 3, Sec. 3] is that some facts are not proved, but only formulated with references to experimental data. Several authors (see the monograph [4, Chap. 1, Sec. 2.5] and the paper [5]) studied the Cauchy problem and initial boundary-value problems for the linearized version of Eq. (1), which has the form