mplayer — multiple sound cards: select audio device 
Mplayer is the ultimate media player. Bold statement, huh? Well mplayer is loaded with features, options, switches and other functionalities. If default settings satisfy your needs then that is great. However, if you have to use some of the non-default functionalities then mplayer's shell syntax can take a little getting used. This post contains examples of how to select an audio architecture that mplayer uses (e.g. ALSA, OSS, etc.) as well as how to select a specific audio device if you have multiple sound cards.
Typically, to play some mp3 audio files with mplayer the following command can be used.
mplayer *.mp3
However, what if you wanted to be a bit more specific, f.e. as to which audio architecture is to be used, e.g. OSS or ALSA. Well, the following commands achieve exactly this, respectively.
mplayer -ao oss *.mp3 mplayer -ao alsa *.mp3
Going further, if you had multiple audio devices you could specify which device to use. For example, to explicitly specify the default device either of the following commands can be used.
mplayer -ao oss:/dev/dsp *.mp3 mplayer -ao alsa:device=hw=0.0 *.mp3
Or, to specify a non-default (secondary) device either of the following commands can be used.
mplayer -ao oss:/dev/dsp1 *.mp3 mplayer -ao alsa:device=hw=1.0 *.mp3
The syntax in the above examples is quite obvious, however at first it can take some effort to get it right.
Enjoy! 
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December 2nd, 2008 at 10:14 pm
that was exactly what I was looking for, thank you very much!
April 25th, 2009 at 6:19 pm
great, since i have 2 sound cards in my "music" server it's a great solution
June 25th, 2009 at 2:35 pm
Thank you so much. Been looking for this for forever, searched thru the mplayer man page etc. but just couldn't find it (using secondary device). Major kudos.