It looks that your mplayer is trying to use a legacy OSS device (/dev/dsp). Applications in modern Linux distributions normally use ALSA, or pulseaudio (which is a layer on top of ALSA).
Here are some ideas:
- Probably you already did this, but if you haven't try steps described in Edison Audio Setup Guide: http://download.intel.com/support/edison/sb/edisonaudio_332434001.pdf . Especially pay attention to commands' output. Compare it to output on your board...
- You can try specifying a particular ALSA device to mplayer. E.g. mplayer -ao alsa:device=hw=1.0. You can also change the default sound driver for libao. It can be set in /etc/libao.conf, using a line like "default_driver=alsa" (no quotes needed). Refer to libao documentation for more info...
- Try running "cat /proc/asound/cards", or "aplay -l". Make sure your board actually has an audio playback device
- For testing purposes try using aplay instead of mplayer for playing WAV files. It is a simpler program that uses ALSA directly, it won't play compressed (e.g. mp3) files though.