One of your problems stems from the calibration value, which I assume is in the unit dB re V/µPa. First, you simply need to change the sign of this calibration value, so that it becomes 176.2 dB re µPa/V. If the signal 'xbit' is in Volts, then you are left with units of pressure after you multiply the 'calibration correction factor' to your signal:
cal=-176.2; % dB re V/µPa
cal=-cal; % dB re µPa/V
cal=power(10, cal/20); %calibration correction factor, µPa/V
calxbit=xbit*cal; %apply calibration, Unit is µPa.
Secondly, it looks like you want 50% overlap, but the overlap input should not be a ratio such as 0.5, but an integer such as round( 0.5*length(window) ).
Lastly, if you want to compute the PSD for each wave file, you need to move the pwelch-line into the for loop (see code below).
If you instead want to perform the pwelch routine for all the audio data, you need to concatenate the 'calxbit'-signals - currently you just overwrite the 'calxbit' and 'pxx' variables in every loop.
path=('C:\Users\datafolder');
d=dir(fullfile(path, '*.wav')); %list all .wav files in path folder
files=length(d); %number of files in folder
% pwelch parameters
nfft = 1024 ; % FFT size
window = hann(nfft); % define the window
noverlap = round(nfft/2) ; % an integer
for i=1:files %for each file
disp(d(i).name);
filename=fullfile(path, d(i).name); %get full filename
[xbit, fs]=audioread(filename); %and then sampled data and sampling
%freq (fs) from audioread function
xbit = detrend(xbit) ; % remove DC
cal=-176.2; % dB re V/µPa
cal=-cal; % dB re µPa/V
cal=power(10, cal/20); %calibration correction factor. µPa/V
calxbit=xbit*cal; %apply calibration, Unit is µPa if xbit is in units of Volts.
[pxx,f]=pwelch(calxbit, window, noverlap, nfft, fs,'psd'); %
PXX{i} = pxx ; % Save each estimate in a cell so it is not overwritten in each loop
F{i} = f ;
end
