Realtek's HD audio sound solutions include the Realtek HD Audio Manager application. This application will assist you in completing basic tweaks to your sound setup, but it does not offer much in the way of deep customization. In fact, there is no way to reset the sound card to its default settings through the Audio Manager. Instead, you will need to manually uninstall and then reinstall the device drivers.
Visit the official Realtek website (see Resources) and download the correct set of audio drivers. Make sure to select the set that matches your operating system (Windows or Linux, for example) and its architecture (32-bit or 64-bit).
Press "Windows-W" to open the Search pane. Type "sound" and click the "Sound" menu.
Right-click the "Realtek High Definition Audio" entry in the "Playback" pane. Select "Properties."
Click "Properties" in the following window. On the next pane, click "Change Settings."
Click the "Driver" tab. Click "Uninstall" and confirm by clicking the check box and selecting "OK."
Restart your computer after the drivers finish uninstalling. Open the Realtek drivers executable file you downloaded from the Realtek website and run the installer to re-install your Realtek drivers. When the installation is complete, your sound will be restored to its default settings.