I would suggest building your project as a battery-powered portable device, with a microphone input (instead of hard-wiring to your stereo), and LED pattern outputs. You can start to learn the basics of making such a device by building a couple of kits:
Use something like the EBAA-1 "Audio Amplifier" kit to lear how to amplify a microphone's output to get useful voltages, and use something like the VM-110 "AC Voltage Monitor" kit to display the amplified audio.
Once you start to get the hang of the basics, you could build a multi-channel audio analyzer, with columns of LEDs that correspond to different audio frequencies -- kind of like a simple LED-based audio spectrum analyzer. Fun for music, and also for talking into and singing into, etc. Have fun! -Mike-