[AO OSS] audio_setup: Can't open audio device /dev/dsp: Device or resource busy
You shouldn't be using OSS, any application using OSS locks the audio device for itself. You need to configure your applications to use either ALSA, ESD, arts or any other thing. I just disable OSS on my kernel, since ALSA can still emulate it for applications that can only work with OSS.
Mplayer can use alsa implicitly if you tell it this way:
mplayer -ao alsa videofile.avi
But it should just use it instead of oss if it's available, so maybe there's some problem with your kernel or there's just some other program that's implicitly using oss.