The power dissipation in the regulators is a function of the loads (presumably some sort of audio amplifier chip) which the regulator feeds. If that is the load, the current will low at zero or low volume, and will increase as the volume is turned up. You will have to measure the current being drawn by the amplifier. Temporarily feed it from an adjustable lab power supply set to 24V, and check the current draw at various listening volumes. When you know that, you will be in a position to design the heatsink requirements for the regulator(s).