ALSA stuff.
Avoid OSS, as it seems the hardware gets locked during use.
aplay -l
to list the playback devices connected. There will be card numbers,
devices and subdevices.
mplayer addresses them as,
mplayer *.mp3 -ao alsa:device=hw=0.1
where this addresses card zero, device 1. At least, I think so.
alsamixer -c 2
will allow device 2 to be adjusted
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At work:
mplayer -ao alsa:device=plughw=1.0 *.mp3
works, playing back from the usb DAC. :)
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Now, configure the mixer (mplayer level control) to effect the second device rather than internal.
.mplayer/config
#ao=alsa:device=hw=1
#mixer=/dev/mixer1
