Acoustic flux amplified from thermal background in semidegenerate n-GaAs at liquid-air temperature is accompanied by electric fields that initiate impact ionization. When the acoustic flux is concentrated in a narrow domain the interplay between the impact and the acoustoelectric effect brings about current oscillations of frequencies in the MHz range that increase monotonically with the net gain factor of the acoustic flux measured prior to the onset of impact. When wide stationary acoustic domains are generated in such samples, the impact causes an increase of the saturation current typical of the acoustoelectric effect. For initially applied fields higher than the threshold for impact, current modulation is observed when the acoustic flux is still in the embryonic stage and its incubation time is increased. The threshold field for impact in epitaxial layers is about 1500 V/cm, while in bulk samples it is much higher. At high power levels these instabilities may bring about the formation of current filaments that burn channels across the samples. The experimental results and a semiquantitative analysis of the instabilities are presented in detail in this work.