[shiroiuma]

>The human mouth is capable of pronouncing all human sounds, it is only a matter of practice.

This is only partially true: the language a person speaks affects the development of facial muscles used to make those sounds. So trying to speak a different language that uses different muscles can be very difficult. Of course, as you said, it's a matter of practice: with enough practice, you can develop those muscles, just like pumping weights in a gym.

[anadalakra]

This is 100% correct. Muscles that aren't used in your mouth/articulators (because your native language doesn't require them) atrophy over time. It is possible to strengthen these muscles with isolated practice -- on BoldVoice we have coach videos where they give you "reps" of muscle movements (such as 20 tongue-ups). We call it "a gym for your mouth" :D It's hard work, but it's the best way to get results.