~$ speaker-test -D plug:surround51:1 -c 6

speaker-test 0.0.8

Playback device is plug:surround51:1
Stream parameters are 48000Hz, S16_LE, 6 channels
Using 16 octaves of pink noise
ALSA lib confmisc.c:1107:(snd_func_refer) Unable to find definition 'defaults.pcm.surround51.device'
ALSA lib conf.c:3493:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib confmisc.c:242:(snd_func_getenv) error evaluating default
ALSA lib conf.c:3493:(_snd_config_evaluate) function snd_func_igetenv returned error: No such file or directory
ALSA lib conf.c:3951:(snd_config_expand) Args evaluate error: No such file or directory
ALSA lib pcm.c:2102:(snd_pcm_open_noupdate) Unknown PCM surround51:1
Playback open error: -2,No such file or directory