Some days ago, my desktop PC would not play sound anymore, although no sound errors were logged in dmesg or /var/log/messages. I always got:
Today while debugging I found out that root could actually play music without problems. At first I thought of a permission problem, but it turned out that all users were in "audio" group and files in /dev/snd/* were read- and writable by that group.
After asking in #alsa whose people insisted that it's a Gentoo problem, I was forced to find the solution myself the hard way. First stop was strace which told me no real errors, except that it hinted to me that the last thing before failing was opening /etc/asound.conf and trying to open ~/.asoundrc. Alsa failed to open the user defined file... but not for user root! It existed there! And I really had "mmap_emulation true" in my asound.conf!
Removing root's .asoundrc gave me the same errors as the normal users got, and removing that offending mmap_emulation line in /etc/asound.conf fixed all that sound problems.