Do a search for MOSFET Amplifier, I have one posted that runs from 12V and is quite loud thru a 4 ohm speaker from www.mpja.com
A logic level N-channel MOSFET, 220uF cap, couple of 68 ohm resistors are all you need. I have the resistors wired with a switch so I can have them in series or in parallel to act as a simple hi/lo volume switch. The 220uF is definitely needed to let the AC out to the speaker.
If you put in the 9V regular you will give up power output: Pmax = V^2/R, so 9V vs 12V makes big difference.
But you will keep car alternator noise out - guess its a tradeoff.