Is it possible to "pimp up your aircraft" with an audio system?
While there are many great reasons not to do this it would seem from a cursory scan of the FARs that the answer is "Yes, you can." -- At least one company (Alto Aviation) is producing aircraft entertainment systems with relatively "pimped out" audio capabilities.

These systems seem more geared toward "cabin class" aircraft where the crew and passenger compartments are separated (mainly business twin/turboprop/jet aircraft), though I suppose you could conceivably install it in something like a PA-32 or Cessna 206, or even light piston singles if you wanted to.  

Such an installation would be subject to the same restrictions as any electrical device installation (requiring a check to ensure it doesn't overload the electrical system, and verification that it won't interfere with navigation or communication equipment), and I would imagine some of the work required may qualify as a "major alteration" as a change to the "basic design of the electrical system" may be involved.
There may also be additional operating restrictions placed on the installed equipment (like "Not permitted for use during taxi, takeoff, or landing" to prevent it from being a distraction/safety hazard during critical phases of flight).