This really sounds like your car is completly dieing..... you need to have someone check for current draw..... it's possible that you have some sort of major power draw and it's killing everything.
You should immediatly disconnect all audio amplifiers and and or any wiring going to the audio system.
Alternatively if you have a fuse that links the audio system to the primary power system, remove it ( make sure you disconnnect the battery first)
If you have a car audio problem or an issue with a second battery it's possible that you've pulled too much current and damaged the alternator
First off.... measure the voltage of the battery before you start up the car
Then start up the car and see what the voltage is.
Then shut the car down and monitor the voltage for the 5 minutes after shutting down.
If you have a mulitmeter.... check and see if you get anything accross the battery terminals if you switch the multi meter to AC
I'm almost wondering if your charging system is putting out a partial AC voltage instead of a straight DC voltage.
Basicly the alternator creates AC voltage and it's output converted down to roughly 14.4 DC volts.
If the regulator/rectifier circuit is damaged then it's possible that it might be putting out partial DC and partial AC
It's kind of complicated to explain.... you will just have to trust the fact that I'm an electronics engineering tech.
Basicly AC/DC conversion circuits can have ripple ( AC voltage on top of DC voltage ) if the current draw is higher than a specific value.
The battery has a "capacitance" but it can only filter out a certain amount of ripple based upon current draw.
If you happen to know somebody with a DC clamp Ammeter it will allow you to trace any power draw from the battery.
Here's what you need.....
http://cgi.ebay.com/EXTECH-MA120-AC-DC- ... dZViewItem
Basicly you will be able to clamp the meter over the wire without cutting into it.... it measures current through electromagnetic induction.
This type of meter will let you clamp any wire and it will instantly show you how much current is flowing through the wire.... that way you can see where all the power from your batter is draining to.
Chris