The invention relates to audio signal processing apparatuses and methods, such as an audio signal downmixing apparatus (105) for processing an input audio signal comprising a plurality of input channels (113) into an output audio signal comprising a plurality of primary output channels (123) and at least one auxiliary output channel (125) using a downmix matrix D, wherein the downmix matrix D comprises a primary downmix matrix DU providing the plurality of primary output channels (123) and an auxiliary downmix matrix DW providing the at least one auxiliary output channel (125). The audio signal downmixing apparatus (105) comprises an auxiliary downmix matrix determiner (107) configured to determine the auxiliary downmix matrix DW, and a processor (109) configured to process the input audio signal into the output audio signal using the downmix matrix D.