Engine would basically drone as it won't be revving up over and over, it'll just build revs slowly and so it'd just slowly grow louder. For an idea of how it would go, play a video game like Gran Turismo, Forza/etc, put the car in it's final gear and then accelerate.
With no transmission, how's the engine going to sound?
Hmm, I'm pretty sure there's still a transmission in there, as the hydraulic coupling basically replaces the clutch. Only thing I can think is they've basically turned it into a single gear setup so they did remove most of the transmission. The motors handle low speeds, and then combine with the engine at lower revs to provide ample torque/grunt (since using a tall enough gear, they mention 250mph so it's gonna be pretty tall, which would generally mean at lower revs the engine is going to struggle, but here they help that with the motors), and then they rely on the high engine output beyond that.
It's possible that I'm wrong, and I'm guessing that they're using the electric motors as an "e-diff" so they can even drop that and really simplify the drivetrain, effectively making it transmission-less where the hydraulic couple functions as the clutch and gear.
And you'll basically almost never be getting 1800hp (on Autblog they said 1500, although they said the 1100hp engine and 700hp motors, but you'll probably never be getting the max output of both). Seems like you'll be getting the most of the power in that lower intermediate range where the engine should be making a large portion of it's power, and the electric motors are still outputting, but then at higher speeds the engine takes over (so that it doesn't drain the batteries).
I think this is an interesting approach and definitely want to see how it fares in real driving. I'd be concerned with what happens when the batteries are drained or even just very low (would it then put all electricity into the crank motor to help the engine spool up?), as if they're struggling to provide the getup then the engine will be a real dog and might even stall it. But they make it sound like the engine can't even get coupled to the drive below about 30mph as the revs of the driveshaft or whatever mechanism they're using have to get up in order to get the engine to get coupled, and it actually seems to almost rely on the revs to start and get the motor up to speed, although that's somehow tied in with the small crankshaft mounted motor). Seems like if you're doing normal driving (even highway driving) you'll be hitting the batteries the most, but then people aren't going to be using these as daily drivers so it's not really a concern, but even for a track day that'd cause problems, it doesn't seem that the engine is used to recharge the batteries (but maybe it does at like highway speeds and around town driving? They mentioned something about when you're going to a charge station that it'll run on the motors to keep from wasting gas from the engine).