Recent sound problems (for example no sound when watching Youtube videos in Firefox or Thunderbird crashes whenever it pops up a calendar alarm) are probably caused by missing plugins. ALSA_PCM_PLUGINS controls which plugins are built; by default, it should build all of them, but apparently a recent update does not set the variable.
Symptoms are, for example, no sound from flash videos within Firefox. When started from a shell various error messages are printed, such as:
ALSA lib pcm_misc.c:740:(snd_pcm_parse_control_id) Cannot get index for 0 ALSA lib pcm_dmix.c:914:(snd_pcm_dmix_open) unable to open slave ALSA lib conf.c:3510:(_snd_config_evaluate) function snd_func_card_driver returned error: No such device ALSA lib confmisc.c:392:(snd_func_concat) error evaluating strings ALSA lib pcm.c:2144:(snd_pcm_open_noupdate) Unknown PCM default ALSA lib pcm_hw.c:1351:(_snd_pcm_hw_open) Invalid value for card ALSA lib confmisc.c:769:(parse_card) cannot find card ''
Try
# echo 'ALSA_PCM_PLUGINS="adpcm alaw copy dshare dsnoop extplug file hooks ladspa lfloat linear meter mulaw multi null rate route share shm"' >> /etc/make.conf # emerge --oneshot alsa-lib # /etc/init.d/alsasound restart