Sound Signals - How to find max value of periodogram(y) ?

Context: reading .wav files of child vocalizations and trying to map out formants from there.

I have plotted the sound signal and used periodogram(y) to get the output energy spectrum graph.

We're testing ranges for formants, so I have predefined maxes.

1. What is the y-label for source signal? --> dB?
2. What is the y-label for the power spectral density- periodogram(y) plot? is that just magnitude || intensity?
3. audioread only reads y, how do I loop through and find the x values that go with

Code:

%% find local maxima in the following ranges

%F1 range
for x1=200:800; %frequency range considered
    F1=max(y);
end

%F2 range
for x2=800:1800; %frequency range considered
    F2=max(y);
end

%F3 range
for x3=1800:3500; %frequency range considered
    F3=max(y);
end