An audio/video file is in a particular format (MP4, WEBM, MKV, AVI, ...) and contains an audio and a video stream. Each of these streams is encoded in a particular format (video: h264, vp9, ..., audio: aac, vorbis). It is not uncommon that a particular player doesn't know how to deal with a particular encoding. For instance, if your player doesn't handle Vorbis and this encoding is used in the file, it will play the video without the sound.
VLC is a player, an application that displays to the screen and sends to the speakers the content of an audio/video file. VLC is a good and free player, known to support almost any format and encoding.
The aggregation takes CPU, in particular without a registered version because the video stream is re-encoded (this is how you get the watermark). With a registered version, there is generally no video re-encoding, which takes ~100 times less CPU.