I believe PreRecorded Announcement on mask deployment is customer option. On many carriers it'd probably be better NOT to have one as the pax would probably freak out even more...
A380 oxy is not chemically provided, it's a gaseous tank system a la 747 (at least on the largest 380 operator it is) Time available varies with several factors but it's enough to get the job done if you're over the Himalayas...