DisturbedAgent1

I ran into the same exact issue. I believe it's due to the fact that OpenIV was updated last week and some of the audio files in the vehicles.awc have been renamed from the hex number they used to be (0xSOMETHING.wav) to actual names. I found that the main siren is now SIREN_PA20A_WAIL, the airhorn became AIRHORN_EQD and the rest are still SIREN_2, AMBULANCE_WARNING and POLICE_WARNING. What you should try is get the vehicles.awc file, extract it to open formats to your desktop and then take your custom siren and before dropping it in there, rename 0x0D329446.wav to SIREN_PA20A_WAIL.wav and 0x0EA58C7C.wav to AIRHORN_EQD.wav and then make sure that when you drop them into the vehicles folder you get a prompt to replace all of the ones you're dragging (usually it's the 5 I listed).