$ speaker-test -c6 -Dplug:dmix6 -twav

speaker-test 1.0.13

Playback device is plug:dmix6
Stream parameters are 48000Hz, S16_LE, 6 channels
WAV file(s)
ALSA lib pcm.c:2145:(snd_pcm_open_noupdate) Unknown PCM dmix6
Playback open error: -2,No such file or directory