I agree, that the terasic demonstration projects have incredible bad documentation quality. But the DE2_i2sound project is almost trivial, a few 100 lines Verilog and a small schematic top level. The WM8731 setup can be found in CLOCK_500.v, there's simply a list of commands (address + value) written to the codec, by changing the data you can set a different configuration. DE2_i2sound is operating the codec in bypass mode, just routing input to outputs, the digital audio interface isn't used at all.