Why is my audio out of sync with the video?
Because audio and video are processed along different paths with different delays, and unless something actively compensates, they drift apart — and the direction of the offset tells you where the problem is.
Why video is usually the late one. Video processing is computationally expensive: scaling, deinterlacing, motion smoothing, noise reduction and HDR conversion all take time. Audio processing is comparatively trivial. So the picture lags the sound, and the more processing a display applies, the worse it gets.
The common causes:
Television picture processing. Motion interpolation and image enhancement introduce substantial delay. Switching to Game mode or PC mode disables most of it and frequently resolves the problem immediately — this is the single most effective fix and it costs nothing.
Separate audio path. Sound going to a soundbar or receiver while video goes to the television means two independent processing chains. HDMI ARC and eARC carry lip-sync correction data for exactly this reason, and most receivers and televisions have a manual audio delay or lip-sync adjustment.
Bluetooth audio. Wireless transmission adds encoding, transmission and buffering delay — frequently well over 100 milliseconds, which is plainly visible. Low-latency codecs exist but must be supported at both ends. Bluetooth headphones for television are a common source.
Streaming buffering, where a network interruption causes a resynchronisation failure that persists until restarted.
Underpowered playback devices dropping video frames while audio continues.
Recording faults, where the file itself has drifting sync — usually caused by variable frame rate footage, and it increases over the length of the recording, which is diagnostic.
How to isolate it:
Does it affect all sources, or one? All sources points at the display or audio path; one source points at that device or app.
Does it affect all content on that source? One file suggests the file.
Does it get worse over time? A growing offset indicates a frame rate mismatch rather than a fixed delay.
Try the television's own speakers to isolate the audio path.
The fixes: picture processing off; manual audio delay adjustment; wired audio; restarting the app or device; and re-encoding variable frame rate files at a constant rate.