Amplify offset voltage

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^{simulate this circuit – Schematic created using CircuitLab}

R5 & C2 filter noise from your 5V rail. You may need to increase the value of C2 if you are getting excessive noise.

R1 & R2 set the gain. Gain is set to 11 with the values shown.

R1 & C1 set the low-frequency response of the amplifier. Increase C2 if you need a lower break-point.

C3 & the parallel resistance of R4 & R3 also set the low frequency break point of the amp. Note that R5 doesn't affect the low frequency response because C2 keeps the node of R5, R4, C2 at AC ground potential.

No, you do not need to supply a voltage to VMID; the voltage is supplied by an internal voltage divider which drives VMID to a voltage midway between AVDD and AGND. This internal circuitry is shown in the upper-left corner of the block diagram on page 1. (This kind of block diagram is only an approximation or a summary of the real internal circuitry, but gives a good overview.) The electrical characteristics table on page 11 "Analogue Reference Levels" lists VMID pin voltage VVMID as typically AVDD/2 and the resistance of the divider RVMID as typically 50Kohm. (Resistors inside an IC are implemented with thin or thick films, with nearly constant ohms per square within the IC. So as long as both resistors are the same width and length, they will have equal value. So the AVDD/2 midpoint will most likely be very accurate even if the RVMID resistance has some variation.)

As mentioned in the question, external bypass capacitors between the VMID pin and the lower supply rail provide a lower-impedance path for higher-frequency noise. These are clearly indicated on page 60 under "Applications Information / Recommended External Components". The 0.1uF capacitor should be ceramic (for low ESR) and must be located within about 5mm of the WM8731 -- if it's too far away, it will not be effective because of too much PCB route inductance. For the same reason, it's best to avoid routing high-frequency bypass capacitor routes though vias, because PCB vias are inductive.

Also note that the schematic on page 60 shows the ADC analog inputs as well as the DAC analog outputs are AC-coupled through a series-connected 1uF capacitor. So there is no need to add offset to the inputs, the DC offset is removed. This codec is meant for audio applications, it's not intended to be used for setting or measuring DC levels. The electrical characteristics table specifies dynamic parameters (SNR, THD) but does not specify any static parameters (INL, DNL, offset voltage, gain error).