Listen at the beginning, middle and end
Play all three corrected previews. Check the same kind of visible event at each position, and avoid comparing an on-screen speaker with unrelated background sound.
If the middle is out of sync but both ends are correct, the drift may be non-linear. This version does not apply separate corrections to different parts of a video.
Keep the source and check the output
Save the repaired MP4 to a separate file. Open it after processing and check playback, duration and sound. Do not delete your source just because the export completed.
For supported H.264 inputs, SyncMend copies the video stream and processes the audio into AAC. This does not mean the entire output file is byte-for-byte identical to the source.
Understand a reserved credit
A full export on a credit license reserves a credit before processing. A successful export confirms usage. A known failure or cancellation normally releases the reservation.
If the app closes unexpectedly or loses contact with the service, the outcome may need review. Keep any output file and follow the recovery message. Repeated exports or deleting profile data can make diagnosis harder.