It looks to me that the line
audfile=strcat('C:\toolbox\audfile\',audio_files(i).name);
is where you get a song. Since the loop variable, i is how you are currently getting the songs you can instead use it to index your random sequence. So, you can put it before the loop and index it to get the random values you want.
a=randperm(80);
for i=1:length(audiofiles)
audfile=strcat('C:\toolbox\audfile\',audiofiles(a(i)).name);
I removed everything else from the randomizer code you posted since I am pretty sure it is unnecessary.
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