Hi
Knowing that the LM386 amp solution works perfectly to solve your sound problem, I would say you should let sleeping dogs lie, and move on to deal with overheating problem, as that's a whole lot easier to fix.
Three pin linear regulators produce heat, as watts, based on how much current is going through them times the
voltage drop across them. You are dropping from 9 volts down to 5 volts, or 4 volts of drop, times the max current of the RassPi, which as stated is 1 Amp. So with 4 volts times 1 amp, you are dissipating 4 Watts in the LM7805. Without heat sinking, it is obviously going to overheat.
Your first and simplest option, is to try and heat sink it better. But this is not a great solution, as you need it in an enclosed space. Second is to try and drop some of the voltage in an external part before it gets to the regulator. Something like a resistor, transistor, or zener can be made to do this. But this is also not the best solution as the added part will just be taking the heat burden in the LM7805's place.
With the above in mind, why don't you use one of TI's "SIMPLE SWITCHER®" products, or something similar? They are very simple, easy to use buck converters in a TO-220-5 package with a minimal need for external parts. They are almost exactly like their three terminal cousins. The difference is these guys operate in switching mode, so they have almost no power loss, and thus, virtually no heat production. Doing this side steps the whole overheating issue, and has the added benefit of being more power efficient. Win-Win honestly.
Something like the
LM2575T-5.0 would be a perfect fit, and is only $2.50 +S&H. It only needs 4 external parts, two you are going to have to use for a three terminal regulator anyway.
*HERE* is the data sheet. Note that C[SUB]out[/SUB], like most SMPS output caps, needs to be a low ESR type or it will fail prematurely. You
CAN get away with paralleling a bunch of normal electrolytic caps together, but you probably shouldn't.