The amplification of sound in piezoelectric semiconductors and semimetals has been studied by several authors (1,2). These studies have indicated that a type of traveling-wave amplification occurs when the drift velocity of the electrons is made to exceed the velocity of sound in the material. For a given drift velocity, the magnitude of the effect is inversely proportional to the scattering time, and hence, amplification appears to be collision-induced. Upon further examination of the properties of such systems, we have found that a second type of amplification can occur for certain electron densities. The second form of amplification is related to the existence of a plasma instability that is most pronounced when the ratio of drift velocity to sound speed exceeds unity by the ratio of plasma frequency to sound frequency. An analysis of both amplification processes is carried out and their relative significance is discussed.