Mean spectra of multiple files

Hello, I would like to count mean spectra from four wav files. Every single registration cotains four data gathered by four hydrophones. I have cut them, so they have the same length. What I want to do is to take five single registrations and create an average spectrum of them (am I right that I have to make separately mean spectrum for each hydrophone?). I have saved five files, and each of them have signal in dB. Now I have to make some operations that will count a mean spectra. Any help?

[Walter Roberson]

Is the average to be taken over the 5 for each of the .wav, ending with 4 averages? Or is the average to be taken over all of the .wav, ending with 5 averages?

[Star Strider]

Context: http://www.mathworks.com/matlabcentral/answers/283989-average-sperctra-of-audio-signals