Simply put, V-Twins are predictable, modern inline's are two faced. (due to low stroke, high compression).
The V-Twin typically has the low RPM torque and a predictable power band, trailing off sooner than an inline. They have a very low, thunderous bark that sounds like a freight train coming up a twisty rural road. They sound so low, unchallenged and beastly when one flys by your house at 80 it's very unexpected because it's a very misleading sound.
The inline is two faced, and has a power curve similar, but not as blatant or violent, to a two-stroke MX bike. They are very doggy until thye rise to a predetermined RPM when secondary fuel injectors come in, air box volume increased electronically, etc. and they howl to high RPMS like 14,15, and 16,000 RPM. MotoGP bikes are V4's, and they rev even higher. Especially the Kawasaki ZX6RR Screamer. The sound of an inline scream's resonating against a deep forest's trees is also very frightening to innocent bystanders. In my opinion depending on the bike, it sounds like a swarm of p**sed of yellowjackets fyling by.
You want to hear a good sounding bike, YouTube A Triumph Daytona 675 with a Jarine RTOne Pipe(Inline 3 cyl), TL1000R or RC51 (Vtwin) or any MotoGp bike. The New 2009 R1 has a crossplane crankshaft that makes it sound like a V-4 rather than I-4