Brand-new MacBook Pro won't play videos on Safari

I can't get my MacBook Pro to play videos in Safari.

On video platforms like YouTube and BBC iPlayer it just shows the first frame of the video. Occasionally, if I leave it to "run", it will skip to a frame further on in the video, but it's never play any sound and never show any video.

On Facebook or Instagram it will very occasionally play a bit of a video, but generally the same issue.

Our wifi is working fine, and videos are playing no problem on phones, iPad and iMac.

The computer is two months old and did work for a while before this issue arose.