The volume level -30dB shown in the graph is just an average level in
a single bin of FFT. You need to sum volume in all bins to get the final average volume level.
A volume of a white noise bellow 24kHz in the following graph is approx. -36dB + 39dB (= 10*log(23000/96000*32768)/log(10)) = +3dBFS (notice, there is a red value stating RMS +3dB).
FIY:
2 equal loud sound sources -> +3dB
4 equal loud sound sources -> +6dB
8 equal loud sound sources -> +9dB
16 equal loud sound sources -> +12dB
32 equal loud sound sources -> +15dB
64 equal loud sound sources -> +18dB
128 equal loud sound sources -> +21dB
256 equal loud sound sources -> +24dB
512 equal loud sound sources -> +27dB
1024 equal loud sound sources -> +30dB
2048 equal loud sound sources -> +33dB
...
More info:
http://www.sengpielaudio.com/calculator-leveladding.htm