Record when the problem begins
Note whether the display briefly changes format at playback start and whether audio is missing only after that event. Plex’s video settings distinguish Enable Refresh Rate Switching from Refresh Rate Switching Delay. The latter pauses playback to give the display time to complete the switch.
This is different from dialogue remaining consistently early or late throughout a programme. A fixed audio offset changes timing between sound and picture; it does not give the television or connected receiver more time to finish an output transition. Keep those two symptoms separate in the report.
Compare the documented transition setting
Plex’s reference says that if audio does not work after a refresh-rate change, increasing Refresh Rate Switching Delay can help. Inspect the current value and change it deliberately for a comparison using the same programme and output path.
There is no universal delay value established by the existence of that control. The actual television, receiver and connection path determine how a transition behaves. Record the original value and the comparison result instead of copying a number from an unrelated household’s setup.
Preserve useful output evidence
Keep the HTPC version, display model, audio destination and any intermediate HDMI equipment with the observation. If sound works when no rate transition occurs, that is a useful boundary; it does not by itself identify which connected device takes time to settle.
If the delay comparison does not help, return to a known setting and provide support with the exact start sequence and result. Avoid stacking a new audio offset, a decoder change and several display changes onto the same attempt. A controlled transition test makes the diagnosis more meaningful than reporting only that the film sometimes starts silently.