My Worst Sounding list is very short, as it only includes cars that I expected more of. A Miata sounds like crap, imo, but I don't expect it to sound any better.
Viper: As I said before, it's too quiet for what it is, and even if it wasn't, there's really nothing to hear anyway.
McLaren MP4-12C: Might get some heat for this, but I don't care. Compared to anything else in its league, price or performance wise, the MP4 sounds far too..."pedestrian". With a Lambo or Ferrari, you'll hear them before you see them, and you'll know exactly what they are(assuming you're an enthusiast, of course). But the MP4? Might as well be a turbocharged leaf blower. Even at full throttle ripping around a road course, it doesn't sound like anything special.
Nissan GT-R: Yeah, it may be "just" a 6-cylinder, but Porsche has more than proven that a 6-cyl can announce itself with the best of 'em. And considering Nissan specifically targeted the 911 Turbo with the GT-R, I would've expected them to make it sound just as good.