After logging I get the following message: Error initializing sound driver...
Using the null output device
None of the multimedia applications works anymore. I get errors like:
'Alsa device had an error' (Totem)
'Can't open audio' (Xmms)
It worked fine since a while ago
I tried to change the sound engine but it still doesn't work

Can you paste the /sbin/lsmod command output? Maybe your sound module isn't loading.

Also, what kind of changes have you done to your system? What packages did you uninstall, upgrade, etc. Focus on libraries and not user-land programs.

Module Size Used by
parport_pc 28933 1
lp 13001 0
parport 40585 2 parport_pc,lp
autofs4 29253 2
i2c_dev 11073 0
i2c_core 21569 1 i2c_dev
rfcomm 42333 0
l2cap 30661 5 rfcomm
bluetooth 56133 4 rfcomm,l2cap
sunrpc 167813 1
ipt_REJECT 5569 1
ipt_state 1857 1
ip_conntrack 41497 1 ipt_state
iptable_filter 2881 1
ip_tables 19521 3 ipt_REJECT,ipt_state,iptable_filter
dm_mod 58101 0
video 15941 0
button 6609 0
battery 9413 0
ac 4805 0
md5 4033 1
ipv6 268097 8
uhci_hcd 35152 0
ehci_hcd 41037 0
via_ircc 24021 0
irda 129657 1 via_ircc
crc_ccitt 2113 1 irda
snd_via82xx 30081 0
gameport 18633 1 snd_via82xx
snd_ac97_codec 75961 1 snd_via82xx
snd_seq_dummy 3653 0
snd_seq_oss 37057 0
snd_seq_midi_event 9153 1 snd_seq_oss
snd_seq 62289 5 snd_seq_dummy,snd_seq_oss,snd_seq_midi_event
snd_pcm_oss 51185 0
snd_mixer_oss 17857 1 snd_pcm_oss
snd_pcm 100169 3 snd_via82xx,snd_ac97_codec,snd_pcm_oss
snd_timer 33605 2 snd_seq,snd_pcm
snd_page_alloc 9669 2 snd_via82xx,snd_pcm
snd_mpu401_uart 10049 1 snd_via82xx
snd_rawmidi 30305 1 snd_mpu401_uart
snd_seq_device 8781 4 snd_seq_dummy,snd_seq_oss,snd_seq,snd_rawmidi
snd 57157 11 snd_via82xx,snd_ac97_codec,snd_seq_oss,snd_seq,snd_pcm_oss,snd_mixer_oss,snd_pcm,snd_timer,snd_mpu40 1_uart,snd_rawmidi,snd_seq_device
soundcore 10913 1 snd
via_rhine 28361 0
mii 5441 1 via_rhine
floppy 65269 0
ext3 132553 2
jbd 86233 1 ext3


this is the output from /sbin/lsmod
recently I installed xmms. Full package name:
xmms-1.2.7-21.i386.rpm
xmms-mpg123-1.2.7-21.i386.rpm
But it worked fine (except for a disturbing noise caused probably by the EQ config)