In the “Audio Channel Mapping” preferences, allow “File Channel 3 [C]” to be mapped to both left and right outputs so that mono (“center”) channels can be heard through both outputs while playing back in the Waveform Editor. This behavior seems to happen automatically when using the built-in speakers as the Default Output, but changing the Default Output hardware breaks it. The result is that the user is forced to listen to mono signals through either the left or right output, but not both.