Is your television muted, or is the volume turned down?
In TV mode, the volume level is determined by the television’s volume. Check that the television’s volume is turned up.
Are there headphones connected to the Nintendo Switch console’s audio jack?
Even in TV Mode, if headphones are connected to the console, audio will be played through the headphones instead of through the television.
If you can't hear any sound from the television, make sure that no headphones are plugged into your console.
Is the HDMI cable properly connected to the Nintendo Switch dock?
Unplug the HDMI cable and reinsert it firmly to check if this resolves the issue.
Are there any USB devices connected to the Nintendo Switch dock that aren't endorsed by Nintendo?
Connecting unendorsed USB devices to the Nintendo Switch dock may cause no audio to be output.
Have you changed the sound settings on your Nintendo Switch console to "Surround Sound"?
Playing software that's compatible with surround sound (5.1 ch linear PCM) on a television that doesn't support it can sometimes lead to incorrect sound output.
Changing your console's sound settings may produce better results. See here for details.
If the above methods do not resolve your issue, please contact your local customer service centre.