Some people, though very few, can become native English speakers on their own. The people that can divide into two distinct camps, musicians and heavy movie watchers (English movies of course).

Musicians by training are talented listeners and heavy movie watchers, whether talented listeners are not, get exposed to so much real sounding English that they can't help but to learn it subconsciously if in no other way.

Typical adult listeners no longer here sounds as they are but map sounds of foreign languages into the sounds their brain already knows. This lack of accuracy in listening is the primary impediment to become fluent in a language.