For those who want to disable the siren try edit the XML file for els at sound sector:
<SOUNDS>
<MainHorn InterruptsSiren="true" AudioString="SIRENS_AIRHORN" />
<ManTone1 AllowUse="false" AudioString="VEHICLES_HORNS_SIREN_1" />
<ManTone2 AllowUse="false" AudioString="VEHICLES_HORNS_SIREN_2" />
<SrnTone1 AllowUse="false" AudioString="VEHICLES_HORNS_SIREN_1" />
<SrnTone2 AllowUse="false" AudioString="VEHICLES_HORNS_SIREN_2" />
<SrnTone3 AllowUse="false" AudioString="VEHICLES_HORNS_POLICE_WARNING" />
<SrnTone4 AllowUse="false" AudioString="VEHICLES_HORNS_AMBULANCE_WARNING" />
<AuxSiren AllowUse="false" AudioString="VEHICLES_HORNS_SIREN_1" />
<PanicMde AllowUse="false" AudioString="VEHICLES_HORNS_POLICE_WARNING" />
</SOUNDS>
Basically, turn off all the siren sound. Set them all to false, didn't disable the airhorn tho.