First of all,
wavread has been removed in the 2015 & 2016 versions of MATLAB. So, if you plan on using your code in a newer version of MATLAB, I recommend that you use the
audioread function instead.
Now, to answer your question. MATLAB already have a built-in function for plotting the power spectral density (Pxx) of a given input signal -
periodogram. I've taken the liberty to copy a piece of sample code from MATLAB's documentation that should help you plot the Pxx you want (it will contain negative frequencies as well).
All you need to keep from your code snippet are the input values seen here. The rest, like the
axis function, aren't necessary to get the plot you want if you use this
The above function will plot the power spectral density of a given input signal
x with no specified window (
 which just means the function defaults to a simple rectangular window). The final argument is the one that I presume you are most interested in:
freqrange. It can have several different inputs, including
centered. You should try both options and see which output you find more useful.
I hope this helps you!