Correlation between amplitude and power
You're dealing with digital signals. Power hence doesn't have a physical unit, just as the amplitude of the signal doesn't have one – where it originated, i.e. the ADC, it might have had one, but usually, that unit just doesn't matter in DSP and the fact that the digital numbers are proportional with some constant factor to a physical signal is totally sufficient.

If you later need to actually map a digital power to a physical power, you'd just calibrate (ie. measure with a known source of amplitude or power) and calculate that constant factor. 

TL;DR: This is DSP. We don't care about physical units, so (mentioning the) resistance is futile.