Audio and video files don't play

Audio and video files don't play, regardless of source/file type. E.g. On brand-new install of Mac High Sierra, after a period of time, audio or video can't be played, regardless of source (YouTube in Safari or Firefox, iTunes music or videos, Photo videos, etc.). The media files are read, because I can forward video frames and see them statically. Only when I restart my Mac will I be able to play video and audio again. Can't find anything online describing a fix to this.