There could be several reasons why some YouTube videos may have sound initially but then lose it. Here are a few possible explanations:
Copyright issues: If the video's creator used copyrighted audio content without proper authorization or licensing, YouTube's Content ID system may have detected it and automatically muted the audio to comply with copyright laws. In such cases, the video owner may have chosen to remove or replace the copyrighted audio to avoid legal consequences.
Content modifications: Sometimes, video creators may edit their videos after uploading them to YouTube. During the editing process, they might have intentionally removed or altered the audio for various reasons, such as improving the video's quality, fixing errors, or updating the content.
Technical issues: It's also possible that the loss of sound in a video is due to technical problems during the uploading process or a glitch on YouTube's end. Such issues could affect the audio track of the video, resulting in sound inconsistencies or complete loss of sound.
Privacy settings: YouTube allows video creators to choose whether their videos are public, private, or unlisted. If a video's privacy settings have been changed by the owner, it could affect the availability of audio. For example, if a video is set to private, only selected viewers can access it, and others may not be able to hear the audio.
It's important to note that without specific details about the videos you mentioned, it's challenging to pinpoint the exact reason for the audio issues. If you have concerns about a specific video, it may be best to contact the video creator directly for more information.