Can aircraft be modified to have an enormous audio system, like the ones in cars? Has the FAA got any regulations on this (or maybe on in-cabin maximum noise volume?), would power be a limitation for the audio system?
Absolutely not, as it would not be in the sense of security.
- It would block engine sounds (eg you won't be able to hear a rough engine)
- It would block warning sounds, for example stall warnings
- It would block any other abnormal sounds
- You will have problems understanding ATC (and they will have even more problems understanding you)
These are just some reasons out of my head. If you want to listen to music inflight, buy a good headset with an AUX-In socket, those normally reduce the volume as soon as a transmission is incoming.
And apart from the audio volume, it would also put a high load on the battery / alternator, which are probably not able to handle such a high load.
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).