Effects of ultrasonic waves propagating at an angle theta relative to the direction of a DC magnetic field on the linear longitudinal electrical conductivity in non-degenerate semiconductors such as n-type InSb have been studied by using a quantum treatment which is valid at high frequencies and in strong magnetic fields. Numerical results show that the linear longitudinal conductivity depends strongly on the DC magnetic field, the sound frequency, the temperature, and the direction of the propagation of ultrasonic waves relative to that of the field.