DVDdoug:
So...you have to assume full power dissipation at all times.
The datasheet does NOT show that. The efficiency is 80% or more above about 1W. Below 1W, the efficiency falls-off.
But... the output power depends on the supply voltage, which is fixed.
Once you decide on your supply voltage you should be able to calculate the heat that will be generated by the chip. That heat won't vary with the volume/type of the music because it comes from the PWM switching frequency, not the duty cycle.