What is a virtual ground?

Parts with asymetric power supply (e.g. AVdd = 3.3V, AGnd = 0V) expect input signals centered around a virtual ground of typically AVdd / 2 and provide output signals centered around the virtual ground.
If you use AICs without capacitors on the outputs, care must be taken that the virtual ground will never be connected to physical ground (AGnd or DGnd). For example, if an output with virtual ground (at AVdd / 2) of a music player that is hooked up to a car charger and therefore has the AIC ground connected to the car ground is connected to the car stereo, the output must be AC coupled with capacitors and the virtual ground must not be connected to the car stereo ground, which would create a short circuit between AGnd and AVdd / 2.