How to terminate a spmd loop

I have a spmd program as shown below:

function [output,frequency] = spmdfunc(music)
    fid = fopen( 'music.mp3', 'r', 'b'); 
    x = uint8( fread(fid, 'ubit1')');
    
    while ( a+31 <= numel(x) )   

        if( x(a:a+14) == syncword)
            poolobj = parpool (3);
            
                spmd
                    if labindex == 1
                        afr = dsp.AudioFileReader(%Content);
						
                            while true
                                % Content
                            end 
					
                            labSend(2, [])
                            release(afr)
					
                    elseif labindex == 2
                        while true
                            % Content
						
                             if isempty(frame)
			           labSend(3, []);
				   break
			       end
                        end

                    else 
                        while true
						
			      if isempty (frame)			
			          break
			      end
							
                            length = numel(output(1,:));
			      output(1,length+1:PCM_length+1152) = [samples(1,:) samples(3,:)]; 
			      output(2,length+1:PCM_length+1152) = [samples(2,:) samples(4,:)]; 

			  end 
                    					
                    end %labindex
                end %spmd end
            delete(gcp);  
        end 
        a = a + 1; 	
    end

    %Display Waveform;
    figure(1);
    t=linspace(0,208,length(output(1,:)));
    plot(t,output);
    title('waveform');
	
end

It manage to run until the final bit, but it has problem in terminating the spmd loop. Means it could not display the waveform. It went back to labindex 1 and cause the error below:

Error detected on worker 1.
Attempted to access x(7.30154e+06); index out of bounds because numel(x)=7301528.

Any idea on the solution? Thank you.