Question

I want slow playback of videos with tempo synchronization. It is sometimes difficult to match the sounds unless the playback speed is changed when matching the sounds.

Answer 1

Videos work with timecode HH :MM:SS:FF not in bars and beats. Tempo is a way to measure musical speed in the units of beats/min. So any changes to tempo only affect the musical speed, not the absolute timecode speed.
This is how it should be and this is how it is.