From the ALSA wiki
mplayer is a mediaplayer app for Linux/X which can play a wide range of audio and video file formats. It is available at http://www.mplayerhq.hu.
2010-02-12 - How to use an alternate ALSA device
So you happen to have a USB audio device or a second sound card and you want mplayer to use that device instead of the typcial internal onboard audio card. First, you need an idea of the current available audio devices. There are a number of ways to find this information, this is just a simple method from a terminal shell...
% aplay -l **** List of PLAYBACK Hardware Devices **** card 0: Intel [HDA Intel], device 0: ALC272 Analog [ALC272 Analog] Subdevices: 1/1 Subdevice #0: subdevice #0 card 1: H2 [H2], device 0: USB Audio [USB Audio] Subdevices: 1/1 Subdevice #0: subdevice #0
In the example above there are 2 audio devices, a typcial Intel onboard audio device and an external ZOOM H2 USB audio device. In ALSA terms (note the card #: and device #: numbers) the 2 ALSA devices would be known as hw:0.0 and hw:1.0 (an alternate would be plughw:0.0 and plughw:1.0 respectively) so to translate this to what mplayer needs we could use this below to use the external USB device...
mplayer -ao alsa:device=hw=1.0 groovy.mp3
or alsa:device=hw=0.0 to use the first device. You could also hardwire these settings so mplayer would always use them without having to specify it on the command line every time, but, then you would always have to make sure the USB device is available on reboot. Hints; the lsusb command will show available USB devices and arecord -l will confirm the capture (audio in) device names.
% cat ~/.mplayer/config ao=alsa:device=hw=1.0