When a YouTube link doesn't play
YouTube tracks play through YouTube's own embedded player, so when one stays silent the cause is almost always one of the items below. Work down the list.
1. The link was a playlist or a channel
The maker accepts video links only: youtube.com/watch?v=…, youtu.be/… and Shorts. Open the video itself and copy that address. A playlist link is refused with “That's not a YouTube link”.
2. “Video unavailable” inside the player
The uploader has turned off embedding, or the video is private, deleted, or blocked in your country. Nothing on our side can override that. Search for another upload of the same piece; official channels usually allow embedding.
3. Nothing happened when the page opened
That is by design. Browsers do not allow sound before you tap something, so the first page's track is only cued. Press Play once. Page cues select tracks on page changes only when Follow pages is enabled.
4. The player is visible and you'd like it hidden
The web viewer keeps the official YouTube video visible during playback. YouTube tracks are not extracted into audio files, and they require internet access.
5. Age-restricted videos
These require a signed-in YouTube session and often refuse to play embedded. Choose a different upload.
6. The file opens as a plain PDF with no player
The playlist rides inside the PDF as an attachment. If the file was passed through “Print to PDF”, a screenshot, or some compressor or markup tools, the attachment was dropped and only the pages survived. Ask the sender for the original file, or for a link.
7. It plays in the web viewer but not in another app
That app doesn't support MPDF yet: it opens the file as a normal PDF, which is exactly what the format promises. Open the file in the web viewer, or point the app's developer to the spec; adding support is a small job.
8. Sound on a phone
Check the volume, and on iPhone try the ring/silent switch. Bluetooth speakers connected to another device are a classic.