sounds odd, but depends on your setup
Do you have the MP4 file locally? What are you using to play that video (OBS Media Source, VLC, or playing outside OBS, then doing a Window or Display capture)?
And what is your audio path? And have you made sure your audio refresh rates are all the same (not mixing 44.1 and 48KHz)
and is your PC powerful enough for real-time video encoding?
Per the pinned post in this forum
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Lots of possible causes for anomalous/unexpected behavior like if your OS isn't clean, and/or under-powered, or if using a USB mic and have overloaded your USB Root Hub, Or you have under-powered PC and have set up a bunch of audio processing effects/filters (noise suppression, compressor, noise gates, etc) etc... lots of possibilities for problems