I need to find the frequency range of musical audio signal (mp3 files), and also to get a graph of the frequencies, to determine frequency allocation, to see what frequency range contain the maximum audio information. Windows 7.
Plot Spectrum: http://manual.audacityteam.org/o/man/plot_spectrum.html
what settings should I select in Plot Spectrum for this?
See the Manual. Choose appropriate settings for your file. You may want to change to Log axis which is a better representation of how we hear audio, with lower frequencies given more width.