Do you get the same result if you play it in the mpv player?
That video link plays find for me with sound. It is the video entitled "Die Feldgrille - Wie zirpt & hört sie", isn't?
I am using an old version of both smplayer(18.2.2) and mpv (27.2) because I still have the screenshot problems in the latest mpv version (see my post https://forum.smplayer.info/viewtopic.php?f=2&t=12634).
To view youtube. I use the Network- Support for sites- mpv+youtube-dl and in 'Advanced' I put my own Mplayer options code--ytdl-format=22/136+140/18/bestvideo[ext=mp4][height<=1080][fps<=?40]+bestaudio[ext=m4a]/best.
I can view the video in both Format 22 and the DASH version formats 137+139.
I hope that trying the mpv player or tickering with Options helps you. If it does not, when I had a problems years ago, I changed my Audio Output to something else in Preferences-General-Audio.