Electrical – programmable variable resistor

At 100% efficiency, the power drawn from the battery will be exactly the same as the power output to the speaker.

Real amplifiers are never 100% efficient, but a class D amplifier can be pretty efficient. TI claims > 88% for the chip used in the amplifier you are looking at.

You should really look at the tas5630 datasheet.

Here is power wasted vs. power output. It is nearly a straight line. Notice a few watts are drawn all the time, even with zero output:

Another graph to study carefully is this one. Maybe two car batteries in series (for 24V) is not enough voltage to get your desired power output:

You need to use Olin's idea of determining the zero crossings of the signals to get the time delay. Then plug the time delay into Scott's equation to get the phase delay.

The following is pseudo-code. I'll leave it up to you to implement each function since they should either be trivial to implement or you should already have something similar written.

reset_timer();    //reset timer to zero

while(get_amplitude(INPUT_CHANNEL) > 0.0)
{
    //do nothing until input channel crosses zero
}

start_timer();    //start timer counting

while(get_amplitude(OUTPUT_CHANNEL) > 0.0)
{
    //do nothing until output channel crosses zero
}

stop_timer();    //stop timer counting

time_delay_ticks = get_timer_ticks();    //get the number of timer ticks between zero crossings 
time_delay = ticks_to_time(time_delay_ticks);    //need to get timer ticks into time domain

period = 1 / frequency;    //get the period from the known frequency

phase_delay = (time_delay / period) * 360;    //calculate phase delay

It's important to read the documentation on the timer you will be using so that you know how to convert from timer ticks into time.