Using the AC97 Codec on an Atlys Spartan 6 Board
The interface to an AC97 codec is a bit more complicated than straight digital audio. The serial data consists of 256-bit data frames; each frame contains several channels of 20-bit samples. The overall data rate is 12.288 MHz; dividing by 256 gives the sample rate of 48 kHz. Part of the 256-bit frame is dedicated to control messages, e.g. to set mixer registers. You may need to do this once after power up/reset to set the volume.

The AC97 spec is available from Intel. Writing your own master is not unreasonable but it will take some time. You may also be able to find one you can reuse. OpenCores has one. There's a very barebones AC97 controller and some general information about the protocol here.

I can't remember how many of the AC97 registers are standardized. The manual I found online for your board says it has an LM4550 codec. Assuming that's right, you may want to refer to the LM4550 datasheet for a complete list of configuration registers.