My favorite amplifier for quick little jobs like you have here is the LM386. This is an easy to find chip and it is low cost. It was originated by National Semiconductor.

Here is a shematic for an amplifier with a volume control. Note that the sound chip with the differential output does not really need to be converted to single ended before feeding it into an amplifier like the one shown here. The differential output on the chip is a method used to gain twice the output voltage swing across the load than could be achieved with a single ended output. But since you intend to amplifify anyway you can just take one of the outputs and wire it into the amplifier input show here.



This diagram was taken from the web site of quasarelectronics and I truly hope they don't mind my copying it here. They sell a small kit board with this chip on it.

Another thing to consider is that if you are just making a single test setup you might just want to go to the nearby electronics store (Here in the USA it is called Radio Shack) and buy a general purpose amplifier that operates on a battery. These are relatively low cost. I keep two of them in my lab for messing around with audio circuits.

Hope this helps
Michael Karas