Barring actual defects or injuries, pretty much all human mouths can produce the same range of speech sounds. (There are some variations, such as the presence or absence of the alveolar ridge.)
However, not all speech sounds are meaningfully used in every language, and, when learning our first language (as children, usually), we learn to distinguish the speech sounds that are actually significant in that language. Ones that aren’t used tend to get classified as “sounding like” the closest one that is… which can be a problem when you’re later learning a foreign language that does make a distinction.
In English, and a wide range of other Indo-European languages (possibly all of them, but I’m not absolutely sure), there is a distinction drawn between the sounds we know (and perhaps love) as “l” and “r” - we hear, for example, the words “lake” and “rake” as beginning with different sounds, and so we know they’re different words (which makes gardening so much simpler). However, a number of non-Indo-European languages don’t make this distinction - and, as it happens, the difference in actual sound between “l” and “r” is really very slight; we English speakers notice it because we’ve been trained to, practically since birth, but someone who hasn’t is going to have trouble.
So, it’s got nothing to do with Westerners having more flexible tongues, it’s just that English-speakers make a distinction between speech sounds that Korean-speakers don’t. (And, I might add, vice versa, but you don’t see Westerners queuing up for surgery to speak better Korean). I suspect that cutting the frenulum would actually do more harm than good - it’d make your tongue more floppy, hence harder to control, at the front. Of course, if this is done to very young children, they’d probably adapt to it and learn to speak without significant problems - but cutting the frenulum can’t possibly do any good, not to anyone with a normally-shaped tongue, anyway.
(Previews) MrO, what you’re describing sounds like what we phonetics types call a retroflex consonant - with the tip of the tongue bent back. Any English speaker can make that noise, but, because it isn’t part of the “English” set of speech sounds, they’ll tend to perceive it as “just a sort of ‘r’ sound”, and will need a lot of practice and training to use it correctly if they learn Korean. I suppose you could slice out a chunk of the upper surface of the tongue, and stitch it up, so that the tip was permanently bent backwards, but I really wouldn’t recommend that…
I grew up in Korea and I speak English with a perfect accent. I can’t understand for the life of me how snipping the thing under the tongue could allow a person to pronounce R’s and L’s better than someone who didn’t go under the knife. If anything, I’d think it would impede their speech in the long run. ::shrug::