That's 2 different issues.
1/ You purchased a license, you should not have watermark on downloaded/aggregated videos. Open VDH settings > "More" > "Conversion". Does it say unregistered ? If so, click "Register an existing license", copy/paste (CTRL+V) your license key, press Enter.
2/ There are many reasons why you wouldn't get sound on a video. First of all, do you hear the sound when the video plays in the page ? If so, the problem may be that your video player is muted or does not recognize the audio codec of the video. You can convert the video, install extra audio-codecs to your player, or download and install VLC which is good player that recognizes many formats.