[speech,Fs]= audioread(fullfile('C:\Users\User\Desktop\GP\Audio_Speech_Actors_01-24\8\',file(k).name));
After that line, speech will be a matrix with as many rows as there are samples, and as many columns as there are input channels. Mono would have one column, stereo would have two columns.
ptch = pwelch(speech,Fs);
pwelch() is able to do its calculations for all channels in the input. It treats each column independently. The result in ptch will have as many columns as there were input channels.
signalpitch(k,:) = ptch;
That line assumes that ptch can be assigned into a single column. That is not the case if the input had more than one channel.
You need to decide what you want to do in the case where there are multiple input channels such as stereo.
Note: the code also assumes that every input file has exactly the same number of samples, which is not a good assumption.
