Reverse Engineering a Door Bell!

[NivagSwerdna]

I am trying to reverse engineer a door bell design that I saw in a You Tube Video... ()...

The circuit is really just made of a 4017 decade counter and a 40106 hex schmitt trigger.

I've drawn out the part of the circuit for the three inputs (Front Door, Side Door and Rear Door) and it is this part of the circuit I am currently trying to understand.  All gates are Schmitt Trigger inverter (I forgot to put the Schmitt trigger symbol in some triangles!)

I have a number of questions!

When the Rear door is pressed the P4DRV output rings the P4 chime and when the Side Door is pressed the P2 chime.  The 6A6Y and 4A4Y gates look like they shape the output pulse but what is the purpose of 5A5Y?

For the Front door there is an 8 note pattern (driven by the 4017 decade counter) and for this the 4017 needs to get a CLK.. How does the combination of 2A2Y and 3A3Y form an oscillator? (The 0 output from the decade counter indicates the end of the sequence... that should stop the oscillator?)

Any hints appreciated!