The ground on the I2S pins is the digital ground. The analogue groundplane can be seen if you hold the main board up to the light - basically, from the DSP to half-way through the 4231 to the leftmost 10-way internal connector (for the docking sled) is analogue territory. Any GND you get from there is analogue ground.

Both are linked (obviously) but the idea is that it's fairly self-contained, ie no high-current digital loads draw from that plane which keeps it quiet.

Hugo