Hi,
Also i notice that in your schematic you are using a two conductor phone plug to plug into the computer. Most computer outputs are stereo and that would mean you need a three conductor phone plug not a two conductor. If your sound card really does have a two conductor output then you are ok, but most have three conductor phone jacks so i thought i would mention this.
The two conductor phone plug has one ground, one signal wire. The three conductor has one ground, but two signal wires one for each channel (stereo).
BTW how much output power do you want to have for this thing? 1 watt is enough for speakers unless you are looking for a very loud system.
Also, what kind of power source do you have available to run the chip?
An example of a audio power amp chip is the TDA1554 ic chip. I've never used it though but it doesnt look too bad. Takes 14v power supply input or so.