OK, a few words of explanation. Firstly, you need to change the op amp if you want to run the circuit from 12V. The MC33204 only runs up to 12V. Replace it with something like the TL071 from TI. When you state '5" speaker' above, is that the dimension of the actual speaker itself (the round bit that vibrates with the music) or is the size of the casing - the standard 5" tall white plastic speaker that most desktop PCs have, with a jack plug on them?
The circuit above is intended to drive into the 1k input impedance of the standard white plastic speakers that are about 5" high. Inside your this is a buffer amplifier chip that presents a high impedance to the PC, but still drives the 4 Ohm speaker inside. The MC33204 is not designed to drive the 4 Ohms speaker directly. For that you need to add an LM380 (just connect it up as an op amp buffer. You dont need any gain out of the LM380, you just need it for its drive capabilities). You need to connect this to the output of the second MC33204 and its output to the left hand side of C6
Ignore the battery and the diode and all the 'pin' references - these were part of the kit I designed - you just want the op amp and surrounding components.
You need to take the output from your PC into the left hand side of capacitor C2 and connect the PC speaker jack plug to the socket SKT1. You need 2 of the above circuit if you want stereo.
The first MC33204 acts as the low pass boost circuit. The second one acts as a gain/attenuator stage.
If you want to examine the circuit further, you can simulate it is LTSpice. Here is an LTSpice tutorial:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm