Bothering to write the repro for this post got me thinking, if I don't use the format everywhere and instead use the output format for the mixer->output connection, and then add another mixer on the input side so I can do input->input mixer in the output format, and input mixer->sink in my desired format, I can mix N nodes and hear them! However, I get from AU (0x92e7e0b2): auou/vpio/appl, render err: -10,874 (kAudioUnitErr_TooManyFramesToProcess) on the input side now.