Hey Duke,
I think people here are a little stumped on your question, so I will attempt to shed a little light on your problem.
To measure current, there are several methods used. You might want to look into
www.allegromicro.com for their non-intrusive hall-effect based current sensors which will put out a proportional voltage. I believe these sensors can measure +/-100A.
Alternatively, you can use a simple sense resistor. This would go in-line with your power supply and you would measure the drop-voltage across it to calculate your current. (Ohms Law!!). Remember to make sure the resistor is the proper wattage (I^2XR or VI) or it will cook.
I am not sure how you would measure your actual wattage to your speakers.. You have to remember that you aren't getting 100% efficiency in your switch-mode power supply (your 12V to a higher +/- voltage) or your actual amplifier. If you know the approximate value of your efficiency here, then it would be possible to get a rough estimate.
I am not to sure about measuring directly from your speakers, maybe somebody else here can help you with that. I would think you would need to consider the output impedance and maximum power transfer to get this, but I am not positive.
Steve