This is a common practice for 4wd's etc who want to run accessories but dont want to discharge their main battery. Just before I give any advice though, have you considered upgrading your battery? Adding a second is a bit of an investment, few hundred dollars by the time you get the equipment, cable and the second battery. There is also the time running the cable through the car and the potential risks if the power cable was ever to get damaged and short out. You may be better off spending a bit more and getting a better battery?
Anyway if you are still set on the idea of a second battery, you will need to go and purchase a dual battery kit. If you want you can purchase a kit from jarcar and make one up from scratch. The kit basically consits of a switch which can be either manually controlled or automatically controlled. For your application, go for an automatically controlled. This switch will disconnect (or isolate is the word used) the main battery when the car is turned off and leave the second battery (or auxillary battery) to supply power to whatever needs it. In this case the audio gear. You can run the battery fairly low, to a point where it wouldnt normally start the car. But when you go to start the car, the switch (aka solenoid) will switch back to your main battery so that you can start the car without any problems.
The car will charge your main battery back to full and then, because it is automatic, it will charge the auxillary battery.
With a manual system, you need to do the bits that the automatic solenoid would, so you need to remember to switch a big switch each time you need to use the auxillary battery so you dont drain your main battery. Good if you dont use it often, but not good for your example.
Mounting the battery. Batteries cannot be enclosed. They need ventilation to remove the gases that are created when they are used. Just remember that when looking for a spot and make sure that the battery could not ever move once it is in.
If you want to research more look up 'dual battery kit' or something like that for a heap of information. When you make your mind up you can go to a 4wd or camping place and purchase one of these kits. Installation instructions will be included.
You do have the option of simply joining the second battery in parallel, but it is not recommended. This will give you a much bigger combined capacity, but both batteries will discharge to the same level, meaning you may not have enough juice to start the car. It also adds a huge demand on the alternator as it now has to charge twice as much as it normally would.
Good luck, let us know what you decide on.