Is the unique sound that a particular make/model vehicle produces included in the trademark/brand?

It is difficult. Harley Davidson tried to patent its sound, but had to withdraw the application after it went nowhere after years. Competitors pointed out that similar engines made the same sound.

Chimes and alerts are very patentable/trade marked, because they are more akin to a musical composition.

BP has had trouble trying to trademark Pantone 348C, a particular shade of green but other companies have had some success in trademarking colours.

In regards to Harley Davidson v-twins. They do have a unique sound. Most v-twins, use a different/better design. HDs use a one pin crank, whilst others use a two pin crank. The sound is most discernable, while the engine is in idle.

Never heard of that. Any aftermarket change in the exhaust system will change the sound from factory.

I always took the baffles, out of my motorcycle pipes. But welded in a piece of metal, because cops, would stick a rod though the pipes, to check for noise control. The welded in pieces, always kept me from getting a ticket….