Usually this happens when the TV is connected for display only but the audio output isn’t switched automatically.
First check your sound output on the PC: go to Sound Settings → Output device and see if your TV/“Wireless Display” is selected. Sometimes Windows keeps audio on speakers/headphones instead of switching to the TV.
Also, disconnect and reconnect the wireless display (Cast/Project), and restart both devices. In some cases, updating your graphics + audio drivers also fixes the missing sound issue.
If possible, check the TV settings too—some TVs have a separate option to enable audio for screen mirroring.