What's the right scale to represent a frequency spectrum?
FFT scaling for audio signals is indeed difficult. Sometimes the energy is distributed over many frequencies, sometimes the energy is only at few or just one frequency. This can result in a very high "crest factor". i.e the ratio of the peak value to the average power (RMS) value. 

This mainly depends on the the length of the FFT: the longer the FFT, the larger the crest factor will get. 

The best ways to deal with this is:


Use floating point representation and math if any possible
Choose an FFT length that is optimal for your application and not longer than you really need


Other than that, the differences that you see are real. FFT is a metric of "spectral density", and for pure sine waves this approaches infinity, while it is quite low for noise-like signals.