Despite they are not set on mute and although i can hear each track played from the source, yet tracks A1 and A4 can't be heard while playing back from the timeline.
Figured it out, with Adobe Support Representative, who will esclate the issue to his seniors, it is a bug, which can be solved by the following steps:
Duplicating the sequence:
1- From the bin, right click on the sequence and from the menu click duplicate.
2- Do not touch/change the sequence settings. Just duplicate it.
3- Ignore the original sequence and work on the duplicated copy.
you will hear all the tracks and the time line will work smoothly.