Not sure what you are asking. You know, Audio Spectrum can use a mask as a path already... The rest is just understanding some basic compositing principles like creating glows with blurreed duplicates of layers and using mattes to cut away unwanted parts.
The way I do it is to make a solid and place Audio Spectrum effect on it. Be sure the beginning is all the way to the left and the end is all the way to the right, this isn't set by default! and set Side Option to Side B. Make the Audio layer set as the song.
Then make a seperate composition, drag the one with the Audio Bars in it and place the effect Polar Coordinates. You would want to change the Type of Conversion to Rect to Polar, then max the Interpolation to 100%.
Should look like this afterwards:
You may need to rescale and reposition afterwards!
Or (as Mylenium said) you can create a Path for the Audio Spectrum. You just go to the Ellipse Tool and (while the Audio Spectrum Layer is selected) Click and drag (while holding shift) to create a circle. This is called a mask path and it will cut your image, set the Mask type to None. You then go to the effect controls of the Audio Spectrum and change the Path to Mask 1
You audio bars should look like this:
This looks a little better in my opinion because there is no warping of the image.