You can start by uninstalling/reinstalling the sound card driver.
- Go to Device Manager right click on the sound card driver and select uninstall.
- Reboot your laptop once uninstalled and it should reinstall the driver automatically upon restart.
- If it doesn't reinstall the driver automatically, you can download and install the driver from Lenovo Support Site.
- Reboot the laptop once again if the driver has been installed.
- After the restart do test the sound.
- If this doesn't work try a System Restore, bring it back to an earlier date or time before the problem started happening.