It seems complicated but it really isnt. Take one youtube tab, open a video with music, and open another tab with a video that you will watch but mute it. Then play them at the same time (press [space] and quickly switch to the other video you're going to watch)
Click this, use the audio
https://www.youtube.com/watch?v=SmsRN78qMxMClick this, mute it and use the video.
Play at the same time.
Post matching videos and music!