5-12v ~15w simple class a audio amp & build to 2.1 pc speaker
The simple answer is that, for a class A amplifier, you cannot do it. As you've discovered, all the units you've found need more than 15 volts, and there's a very good reason.

The power P dissipated by a resistor R at V volts is  P = V x V / R. Rearranging the terms says that V x V = P x R. Assuming your speaker is a nominal 8 ohms, P x R equals 120. The square root of 120 is 11 volts. So, let's say you have a 12 volt amplifier which can run its output from 0 to 12 volts. Since this is audio, that is AC, you would need to set one end of the speaker to 6 volts, and this would correspond to 0 volts on the input, with an output of +/- 6 volts around this center. The output power would be (for a perfect square wave) 6 x 6 / 8, or 4.5 watts.

In order to get 15 watts, you would need a minimum power supply voltage (for square waves) of +/- 11 volts, or 22 volts. But it gets worse. You'll note I keep saying "for a square wave", but audio is not a square  wave - it's a bunch of sine waves. For sine waves you need an extra factor of 1.414, or the square root of two. 1.414 times 22 equals 31 volts. And this is an absolute minimum, since it assumes that an amplifier running from 31 volts can put out 0 to 31 volts into 8 ohms, and in general this is not true - it can only come within a few volts of this. 

So to sum it up, 5 to 12 volts will never produce 15 watts into 8 ohms with a class A amplifier. Sorry.