Castmate
Chromecast plays the video but there's no sound
Picture on the TV, silence from the speakers. The usual causes and fixes.
Check the obvious first
- TV volume and mute, and the Chromecast's own volume (in the cast controls or Google Home).
- If a soundbar or receiver is connected, its input and volume.
The audio format
Chromecast plays AAC and MP3 itself. Dolby Digital and Dolby Digital Plus (AC-3, E-AC-3) are passed to the TV or soundbar, which has to decode them; if it can't, you get picture without sound. Streams that offer several audio tracks often have an AAC one too.
Picture and sound kept apart
Many sites deliver sound as a separate stream next to the picture (HLS with an audio group, or DASH). A cast that only sends the picture stream plays silently. Cast the master playlist (the one that lists all qualities), not one quality's playlist.
Downloaded file with no sound
For the same reason, saving only the picture stream gives a silent file. Castmate joins the separate sound back into one MP4 when it saves (from version 1.0.5). Some stock video sites (Pexels, Pixabay) simply have no sound track.