A codec describes how audio or video is compressed and decoded. A container packages tracks, timing and metadata together. MP4, Matroska and WebM are containers; H.264, HEVC and AV1 are video codecs. Playback requires a compatible combination rather than a familiar extension alone.
Picture the separate jobs
A container can hold video, several audio tracks and subtitles in one file or presentation. The player first interprets that structure, then sends each compressed track to an appropriate decoder. Failure at either stage can stop playback, which is why changing a filename extension rarely solves an unsupported-format error.
Gather useful information
Use the player’s media panel or a trusted inspection tool to find the container and codecs. Include audio information as well as video when only the sound is missing. A report saying MP4 does not tell support whether the video is H.264 or HEVC, nor whether the audio format is supported.
Understand remuxing and transcoding
Remuxing moves existing compressed tracks into another container without re-encoding them. Transcoding decodes and encodes the media again, potentially changing quality and processing cost. Neither operation should be treated as a way to bypass access restrictions, and a simple remux cannot make an unsupported codec suddenly decodable.