I think that I discovered why few strings are different.
"VEHICLES_HORNS_SIREN_1" - Police Primary
("RESIDENT_VEHICLES_SIREN_PA20A_WAIL" doesn't work for some reason)
"VEHICLES_HORNS_SIREN_2" - Police Secondary
("RESIDENT_VEHICLES_SIREN_2" doesn't work for some reason)
"VEHICLES_HORNS_POLICE_WARNING" - Police Warning
("RESIDENT_VEHICLES_POLICE_WARNING" doesn't work for some reason)
"SIRENS_AIRHORN" - Airhorn
("RESIDENT_VEHICLES_AIRHORN_EQD" doesn't work for some reason)
As you can see, names for Primary and Secondary are SIREN_1 and SIREN_2. In sfx folder there is one more archive: SCRIPT.RPF where we can find SIRENS.awc file with this content:
And from what I see, they have better quality then sirens from vehicles.awc.
I have no clue where can "SIRENS" file be located with AIRHORN but maybe it's also a solution for POLICE4 strings