First, I think there are misunderstandings. You use "copy write" instead of "copyright", and I think you mean to take the RMS value, because taking Square Root of sound seems to be meaningless.
You have to use "sqrt" command, not "_sqrt", Matlab Help is a great thing and I'm wondering why you don't use it.
Anyway, I think Simulink is much better for your job. use File/New/Model and Simulink is at your service. from the toolbar choose "library browser". You need a source from workspace (sound matrix) and a sink to it (result, in the form of a matrix, too), and also an RMS block. search for them in the library browser. Don't forget to see Matlab Help, It's Great, believe me!