I can't move that yellow line on the master track. Any idea about that? I can move it down, but not up. It stays at zero.
The value is in db. 0 db is full volume. Moving it down decreases the volume. Moving up increases volume. The master track is constrained to not go higher than 0. The other tracks are not. If you want to increase the volume, you will need to adjust the individual tracks. Again, expand the selected audio track and set to show track volume or track keyframes.
Which version of Premiere Pro do you have?
The audio mixer works at the track level. Each column in the mixer represents a track in the sequence plus a master track that is the mix of the other tracks. You can apply audio effects in the mixer and the effects will apply to all the clips in the track. The mixer has a lot of capability -- too much to describe in a short entry. Let me know which version you have and I'll see if I can find a tutorial on that mixer for you.