This paper presents the use of envelope tracking in class-D digital audio amplifiers supplied through step-up DC-DC converters as applied in automotive, battery-operated, and portable devices. It is shown that envelope tracking offers significant improvements in efficiency of up to 26% for the class-D stage of a 100 W amplifier in a typical usage scenario, with a corresponding overall system improvement of up to 27% including a boost converter. Detailed simulation results of the complete system are presented, as well as experimental results performed on an existing 100 W class-D amplifier design. Efficiency versus power results are combined with music power distribution profiles to simulate typical usage cases. Challenges relating to envelope tracking and DC-DC converter control are discussed. It is concluded that there is significant potential for envelope tracking to provide improvement over the standard fixed supply amplifier design - especially at low power, where the amplifier is normally used most.