Line out bias on sound card output
I would assume it's applying that DC-bias because it is cheaper to use an amplifier to generate a mid-rail then use proper series-capacitors to decouple the line out signals.

Basically, the line outputs need to be able to swing positive and negative. The general way this is done is with a series capacitor in each line-out connection, which blocks the DC. However, to get low enough frequency response with headphones, which can be as low as 8Ω, you need a very large capacitor, which is expensive/physically-large.

I would guess they have a additional output driver in the IC that simply buffers a resistor divider, and by using that, they remove the need for the series capacitor.

If you need to reference the outputs to a different circuit, you should use series capacitors to block the DC. Ideally, you should have a high-impedance connection to the USB ground, for any DC, and then the local ground of your circuit should actually be coupled to the HP_COM with a series capacitor. 



They actually mention this indirectly in the datasheet: