Thing happens in this way. One day, I re-open my project to have further editing, footages lost because I've changed the project name and place it in other folder, then, I relocated the footages (Don't know whether the happened related to the lost of footages). And when I done, I play the video, it works properly with sound (despite one part of it lost), but I found audio track 1 is lost, just like what I showed you in the picture with a red arrow pointing there. I got mad. So, may I ask, where can I find the 'A1' bar again?
Thanks for answering.
P.S. I guessed the sound was mixed with the picture originally, but finally found the track is actually funtioned without displaying in the timeline, I found this by putting one of the shots into the resource monitor and played it, it has no sound.
Scroll up on the right side.
Hehe, I'm so stupid. I'm sorry. Thank you so much, I got my track backed...!!!