I would recommend you avoid the class D amplifier you linked. It will probably work but I'm not sure if it would sound good. It's designed to drive speakers with a really high frequency switched signal. Some of the filtering is on board, but it depends on the speaker impedance for the rest.
An op amp circuit to do this is probably the best bet. This is a really good basis for the design:
http://www.muzique.com/schem/mixer.gif
Use 1 of the channels and eliminate the other 3. Duplicate this for left and right outputs, or you could do 2 channels to mix left and right out of Bela if needing to go stereo to mono for some reason.
Either way start with something near 22k for R1 to get some extra boost. Decrease R1 further as needed to get the amount of gain required. 50k-100k pot is a good choice if you want it to be adjustable.
Here's a way to modify the circuit for use with a pot:
http://www.faqs.org/docs/electric/Exper/05326.png