unfortunally I'm a noob, but I think I have a solution. It's called "Shifty VU Shield".
It has a stereo input. The left channel is sent to the analgue Pin 2 and the right channel is sent to the analogue pin3 of your arduino uno. You will get int values that you can also map.
I'm using it to control two servos independently. But I think most people use it to make visualisations.
here'S the shield: http://docs.macetech.com/doku.php/shifty_vu_shield
at the bottom theres a light desk example made with it.
another solution (but you have to read and try a little bit) could be "processing". Processing has some examples how to visualize music. You will get results like winamp on your Screen. But it's also possible to to send or recieve the values of the programm via USB (write.port or somethin like that).
it works like this: You play a song on your PC. The "Proccessing" programm isreading out the sound values and making squares or circles or whatever depending on these values. But you could also send the values via USB to the arduino. On the arduino you will need a programm that captures the values and do what you want.
While I was trying doing my project, it worked. But it's not stereo. So you maybe could not use two light bars.
but... using the shifty vu shield is more independent i think. because... the code is on your arduino, you just need a mp3 player and you dont need a PC or Laptop or other software. and... it's stereo