Hi, I hope it will be available soon. But it would also be nice to at least make the police siren available in the Forza Shop. 
FH3 had that too: Car Horns List - [May 9 content update] - #15 by ManteoMax
Thank you and best regards
Dan
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Hi and Welcome, just flicked online, if you’re pc player you can download the ogg. file for the siren Car Horns | Forza Wiki | Fandom I’m xbox, so don’t know if that’s of any help to you, or through ‘Super 7’ events 