You don't say what you have tried but these are the first things to check.
Have you run the audio troubleshooter?
Start - type in audio troubleshooter - find find and fix audio playback problems- next.
Do you have any headphones plugged in?
Try looking in Device Manager to see if drivers need to be updated.. Start - type in Device Manager - under sound video and game controllers select your sound card, open it, select driver tab, update driver.
If this doesn't work in Device Manager uninstall your audio driver, restart and windows should automatically attempt to reinstall the driver.
If your audio device is a USB or HDMI Connection have you set that device as default?
Start - type Sound - select sound from results - On Playback select a device and choose default.