In order to fix the issue, go to Start, type dxdiag and open the utility. Click the Sound tab, check your audio card model and download and install the latest drivers available from the developer's page. After that, right-click the Sound icon from System Tray and open Playback devices. Right-click anywhere in the opened tab, check Show Disabled Devices option, click on your sound device and press Set Default button. Also, click Properties and select Use this device (enable) as device usage. If it doesn't work, install Combined Community Codec Pack and try again.
If there is still no sound, it means that your version of Windows Media Player doesn't have DVD playback capabilities. In order to play DVDs on your computer, follow the instructions provided by Microsoft Support team.