How To Merge Multiple Clips In Audacity To One Audio Track

Thank you! I prererecorded my PPT audio, and noticed some mistake, so some bits I have to re-record, then after cutting, some bits have higher volume, and some bits have lower volume. this 'normalise' function is amazing! Thank you!
Can I ask another question not related to these two functions that you showed? After I export audacity audio into wav and inserted in my ppt, I record the screen, and then inside mp4 file, I noticed the screen is somehow slower than my voice! although during recording I have made sure that the screen goes to where/what I am talking about. Really annoying! any solution to that? And I found my audacity somehow stopped working, can't export wav file anymore, if i click on it, it's just a 1kb empty file.. tried many times and reinstalled.. still not working, but I guess this is a software bug, nothing really to resolve it from customer end.

Thanks coach! Because I literally had no
idea "how to" do it! LOL!