An MP4 file is a container format that holds video and audio streams encoded with different compression standards, called codecs. While widely compatible, playback issues arise if the software or device lacks the specific codec used to compress the video (like H.265/HEVC) or audio (like AAC-LC). This is different from simpler formats where the encoding method is usually fixed. The container itself (.mp4) doesn't guarantee universal playback if the underlying compressed data isn't supported.

For instance, a video shot on a newer smartphone using the HEVC codec might not play on an older media player app or computer that only supports the older H.264 codec. Similarly, an MP4 file encoded with a less common audio format might have sound missing on your TV's media player. Common situations include trying to play files on older operating systems, specific web browsers without plugins, or budget mobile devices lacking hardware decoding.

While modern software and hardware increasingly support common codecs like H.264 and AAC, limitations remain. Files using newer or proprietary codecs require compatible playback software or conversion tools. Corrupted files or Digital Rights Management (DRM) restrictions preventing playback on unauthorized devices are also causes. Checking the file's actual codecs using media information tools is the first troubleshooting step, followed by ensuring your software supports them or converting the file to a more compatible format if possible.