I get asked this question all the time, and perhaps one more attempt at an explanation will help.
Voltage is what determines loudness.
Let me explain.
Let's imagine we have a stereo power amplifier delivering 10v at its output. Now, add two loudspeakers, each with identical sensitivity (meaning that for a given voltage, they will produce the same loudness). One is an 8Ω speaker, and the other is a 4Ω speaker. We connect one speaker to the left channel and the other to the right channel.
What happens? Do they both play the same loudness?
You guessed it. Yes, they both play at the same loudness despite consuming different amounts of power to do it. And this is where I think it's confusing.
The 4Ω speaker needs 25 watts, while the 8Ω speaker needs only half that.
The voltage to both is the same.
In order for the amplifier to produce the same voltage into differing loudspeaker impedances, we need more or less current (amps). The lower the impedance, the greater the current that is required to maintain the same voltage.
It is the voltage that determines loudness. It is the current that changes in order to support the voltage.
Hope that helps.