If you are running Ubuntu 11.04/11.10 and your computer stops making sound for no reason, then I suggest that you follow the instructions given below to restore sound back to your computer.
Before we start, you should know that these solutions are some from many solutions available to fix sound issues. So, they may help some users, and some not.
Unmute Sound With AlsaMixer
If AlsaMixer it is not installed, then install it with this command:
sudo apt-get install gnome-alsamixer
2. Using your keyboard arrow keys (Left/Right/Up/Down), unmute PCM and Front:
3. The sound should now work fine. Press Esc to exit AlsaMixer.
Restart ALSA Audio Driver
If your audio device is not listed at all in the Hardware section, then try to restart your ALSA audio driver with this command:
sudo alsa force-reload
Note: Sound may disappear after rebooting your system.
Reinstaling Missing Sound Modules
If your audio device is not listed in the hardware tab of your sound manager, then your sound modules are not installed. To fix this, run this command via the terminal to install missing sound modules:
sudo apt-get install --reinstall linux-generic
sudo aptitude reinstall linux-generic
Reboot now your system. The sound should now work fine.
Onboard Realtek ALC887
If your motherboard has an onboard Intel HDA audio (ALC887), then follow these instructions:
1. Open the Terminal and run this command:
sudo gedit /etc/modprobe.d/alsa-base.conf
2. At the end of the file, add this line:
options snd-hda-intel model=generic
Save the file and reboot your system.
Note: Also applicable for Ubuntu 12.04/10.10/10.04.