Please check that:
1. You have the correct output plugin selected.
2. No other programs is blocking the soundcard.
3. Your soundcard is configured properly.
yongyi@yongyi:~$ sudo gedit /etc/asound.conf
- using device default
- using device default
ALSA lib conf.c:1566snd_config_load1) _toplevel_:22:2:Unexpected char
ALSA lib conf.c:2811snd_config_hook_load) /etc/asound.conf may be old or corru pted: consider to remove or fix it
ALSA lib conf.c:2672snd_config_hooks_call) function snd_config_hook_load retur ned error: Unknown error
ALSA lib conf.c:3040snd_config_update_r) hooks failed, removing configuration