Lower frequencies stronger in FFT?
Yes, this is very much expected. What you see is a "pink" spectrum, i.e. constant energy per relative bandwidth, as compared to "white" which is constant energy per absolute bandwidth. For pink signals the energy between 1-2kHz is the same as from 2-4kHz (each represents a doubling of bandwidth or an "octave". 

Most natural audio signals (Speech, music, movies, etc.) have a pink-like spectrum. Also most acoustic background noises (microphone noise, HVAC noise, air pressure fluctuations, generic background) tend to be pink or even more skewed towards lower frequencies.

The human auditory systems works that way, too. In the inner ear the audio signals are broken down into bands of constant relative bandwidth (called "Critical Bands") which are roughly the same as a third octave spectrum.

The best way to look at audio spectrum data is to plot it on a logarithmic frequency scale.