Like is a preposition and as is a conjunction.
I have heard what you have as well, but not in those words. The Harper Dictionary of Contemporary Usage states that “probably no single question of usage has created greater controversy in recent years” than the conjunctive use of like. (Per Grammar Girl)
A preposition is a word that ‘positions’ or situates words in relation to one another. (Examples are in, around, beside, under, through, etc.)
A conjunction is, simply, a connecting word. Common conjunctions are and, but, and or.
Use like when no verb follows.
Sarah throws like a quarterback. (quarterback is the object of the preposition)
Andrew acts just like my brother. (brother is the object of the preposition)
If the clause that comes next includes a verb, then you should use as.
Sarah throws as if she were a quarterback.
Andrew acts just as I would expect my brother to behave.
Obviously, if it's a similarity one wishes to express, it's appropriate to use like (It sounds like waves coming to shore.)
Having said all that, the above is a guide to grammar. How you choose to say something (like/as if/as though) is your choice. I don't like how overused like is, so I tend not to use it as much as some.