1. A hearing assistance system for processing speech from a sound source, comprising: a microphone configured to receive an input sound and produce a microphone signal representative of the input sound, the input sound including the speech from the sound source; a processing circuit configured to process the microphone signal to produce an output signal, the processing circuit including a multichannel Wiener filter (MWF) and configured to approximately optimize the multichannel Wiener filter (MWF) for noise reduction and speech quality in the output sound by minimizing a noise variance with constraints formulated using a priori spatial information about the sound source and independent of signal correlation matrix, the constraints ensuring that a measure of speech distortion is below a first threshold parameter and ensuring that a measure of noise reduction performance is at or above a second threshold parameter; and a receiver configured to produce an output sound including the speech using the output signal.