If you are sitting in a normal car like mine, you can usually hear emergency vehicle sirens LONG before you can see them through the minivans and pickup trucks.
Given that lines of sight - even on congested roads - tend to substantially exceed sound transmission distances, no.
Given that some people don't drive 16 foot tall SUVs, yes. Given that some roads are curved, yes. Given that some roads have intersections with buildings at the corners, yes. Given that some roads are flat as a pancake, and straighter than the pole your mom dances on, yes. Given that some roads have hills and rises in them, and ambulances can be coming towards you from over the hill, yes.
There are plenty of situations where you can hear an emergency siren when it's physically impossible to see the vehicle, because it isn't line of sight. GP explicitly stated "in a normal care like mine." I drive "a normal car" too, and I agree with them, that it's sometimes impossible to see around the dolt ahead of you that needs to drive a jacked up Ford Excursion with 1 person in it. The problem is, "a normal car" isn't very normal anymore. The visibility out of my car is great, fuel economy is great, handling is great. But when I'm in traffic with the massive numbers of SUVs people seem to need to buy now, sometimes the only thing I can see is the rear slab of bodypanels of the SUV/minivan/truck ahead of me. Same to the sides, and same behind. I'd rather drive in a bunch of transport trucks sometimes, because I'm low enough that I can see clear under the trailers. My visibility is blocked less by an 18 wheeler than a common-as-dirt Dodge Caravan.
One thing we all need to remember, especially before posting a response to someone else's experiences, is that we're not all the same. We all have different situations, different experiences, live in different environments, and we all, in this case, drive different vehicles. Posting something telling someone they're wrong, when you obviously haven't had the experience they have, just makes you look like a douche.