Re: "Car horns symphonise accompanied by" arrrghhh!
If I'd written "symphonies", the sentence would lack a main verb and therefore not be a sentence at all.
You old fogey you. Don'tcha know that it's cool, hip and froody to not speak Soverign's English any more? Doing so merely marks one as an elderly pedant.
Or so I'm told.
 One never knows when Queen's English might become King's English.
 Come to the Dark Side. We have declensions..