It should always be made by the PIC is was seated in his position during the initiation of the maneuver

If*
His/her position

Who: CM1 (Because the PIC could be in RH seat as Instructor or Examiner)
When: Just after OXY MASK.
ECAM: Once all memory items are done.

My understanding of QRH procedure is that the PM is in charge of making a PA announcement as she/he is reading and doing the procedure.
Also a pre-recorded announcement should be triggered.

The actual procedure is down to the operator. I have seen various EMER DESCENT procedures with Airbus operators (operating various types). The PA announcement was never part of the memory items and generally performed by the PM, but it was at various points in the order. For example some where "EMER DESCENT (PA)... Announce" was right after OXY MASKS and others where it was further down the line (after "ATC... Notify").

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