What's your goal? If you want a simple audio indication connect an external audio amp to a pin on the GPIO header. Then generate a square wave of the desired frequency in verilog. Put a resister in series if needed to avoid saturating the amp. Maybe add an RC filter in for less buzz if desired.
If you want to synthesize and generate a specific tone and output it over the audio jack, you are in for more work. First synthesize a signed 16 bit audio signal as desired. Then convert it to I2S. This goes to the DACDAT and DACLRCK pins on the codec.
You use I2C to initialize and control the codec. See the codec user guide for more info on this. Also, take a look at the DE2_115_Audio demo on the board's system CD.
