Comentário oficial
WASAPI (Windows Audio System API) is only capable of handling one stereo pair of audio outputs, whereas ASIO (Audio Stream Input Output) can handle multiple pairs of ins/outs. Typically ASIO devices / drivers also feature lower latency rates.
For devices like the SB2, you have 2 pairs of outputs; the master, and the headphones. If you use the WASAPI drivers, you'll only be able to output to the first pair - the master, thus rendering the headphones useless.
