Check
the connection at the back of your computer, Sometimes its contact will
be loss. Some computers have speaker connections on both sound card and
the modem.
If you're sound card is not built-in, try to remove it and clean its connections.
Check the device manager, maybe there's a problem with your sound driver.
Try to re install your sound driver.
Then click start > control panel > Sounds and audio devices
Check it if you can adjust the volume or not.
If not, go to start > run
Type services.msc then hit Enter.
When the services will display, double click windows audio
Set the start-up type to automatic then click start.
