Opening a door from inside when all doors locked should unlock all doors-sounds normal.
Key fobs are always playing up so you may have a few faults.
If there is a fault on drivers door lock it won't lock other doors.
Temporarily try leaning in from pass side and lock drivers side. Then lock pass door with key and see if that keeps all doors locked. You may have to unlock from pass side to get in.
For stereo problem and light on buzzer- has it had a different stereo fitted. If it has, it may be wired up incorrectly.
dave
February 2011