I stumbled on a strange problem. I have a cuddly 66s cat video where there is a laud Whatsapp ble ble ble ble ble after the 60s.

I made two videos with Handbreak. One 66s long and one 60s long.

Now when I mux the 66s video with 60s audio in MKVTOolNix, it makes it 60s.

I use ancient version 8.8 on El Capitan.

Where do I uncheck the "truncate to the shortest track" option or how do I fill the last 6 seconds of a 66 second audio with a silence or just lower its volume? Which actually gave me an idea, I will use CoolEditPro for that.