From the datasheet:

2.2.1 Clock Generation

The TAS1020B requires an external 6-MHz crystal with load capacitors and PLL loop filter components to derive all the clocks needed for both USB and codec operation. Figure 4-1 shows the connection of these components to the TAS1020B. Figure 4-1 also shows a ground shield residing on the top layer of the PCB and underneath the crystal and its load capacitors and the PLL components. The PLL is an analog PLL, and noise pickup in these components can translate to phase jitter at the output of the PLL, which in turn can translate to distortion at the codec. A ground shield is recommended to attenuate the digital noise components on the board as seen at the PLL.

The AVSS and AVDD pins on the TAS1020B are used exclusively to power the analog PLL. To maintain isolation from the digital noise residing on a board, AVSS should be a separate ground plane that connects to the primary ground plane (DGND) at a single point via a ferrite bead. The ferrite bead should exhibit around 9 Ω of impedance at 100 MHz. AVDD should also be distinct from DVDD. A recommended architecture is to generate DVDD and AVDD from the same regulator line, with each derived from a RC filter in series with the regulator output. It is finally recommended that the ground shield for the crystal and its load capacitors and the PLL loop filter components be connected to AVSS at a single point via a ferrite bead of the same type as above.

Using the low frequency 6-MHz crystal and generating the required higher frequency clocks internally in the TAS1020B is a major advantage with regard to EMI.

(emphasis mine)

So it's basically suggesting that you should put a copper filled polygon on the top layer underneath the crystal and it's load capacitors and connect it to your analog ground via a ferrite bead in order to provide a low impedance current return path to reduce jitter on the PLL.

EMI shielding

Look at C2/C3. This is a capacitive voltage divider that indeed will make your computer's case carry a voltage of approximately half the line voltage when not connected to ground. When your computer is not grounded and you touch the case you may feel the tingling and it should not reach a dangerous level. The two capacitors are there to improve the casing's performance for high frequency shielding (which a power supply like this as a lot of), so you mustn't remove them.

Safety ground

The other reason for the casing to be connected to ground is the fact that the entire case is made of metal and is easy to touch. Whenever some components fail or a wire comes loose, the entire case may carry line voltage and that can be leathal. When connected to ground, either the internal fuse will blow or an GCFI (Ground Fault Circuit Interruptor) in your home will trip.