Alsa: Device or resource busy

I have seen this prblem a couple of times, more or less randomly:

mplayer 
[AO_ALSA] alsa-lib: pcm_dmix.c:996:(snd_pcm_dmix_open) unable to open slave
[AO_ALSA] Playback open error: Device or resource busy

Today I solved it in this way:

lsof /dev/snd/*
[look at the pid's, excluding kmix]
 kill pid's

I dont' know wether this always works, anyway...