user@deimos:~$ speaker-test -c6 -Dplug:dmix6 -t wav

speaker-test 1.0.11

Playback device is plug:dmix6
Stream parameters are 48000Hz, S16_LE, 6 channels
WAV file(s)
ALSA lib pcm_dmix.c:862:(snd_pcm_dmix_open) unable to open slave
Playback open error: -16,Device or resource busy
ALSA lib pcm_dmix.c:862:(snd_pcm_dmix_open) unable to open slave
Playback open error: -16,Device or resource busy
ALSA lib pcm_dmix.c:862:(snd_pcm_dmix_open) unable to open slave
Playback open error: -16,Device or resource busy