Can I use the Behringer Eurorack UB802 Mixer as an audio interfac.?
The UB802 is a discontinued item. There are a number of good USB and FireWire interfaces out there these days. Unless you just have a UB802 sitting on your desk. In which case, you will have some issues with noise and interference, especially if you are inputting into a laptop. Built in sound cards are a nightmare for recording serious music or voice.
Also, the outputs are not exactly balanced for input into sound cards, you will likely experience a severe amount of distortion in your recording that you can not just filter out.