'It sounds as if Jack has found the perfect job'!
'It sounds like a violin, but I think its a viola'.
Are these the correct forms?
It was drummed into me at school over half a century ago that to say 'It sounds like he's in trouble' is very bad English. You only say 'sounds like' if it is something you can hear.
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.
As if precedes a clause.
He acted as if he didn't know who I am.
Although like and as if are not generally interchangeable, it is acceptable for like to substitute for as if in this situation. In the same way, it links "he acted" with its complementing clause:
He acted like he didn't know who I am.
This usage is common, and compatible with prestige dialects of English. Today, nobody in his right mind will regard you as "uneducated" if you use this.
It's not clear if your grammar teacher's prescriptive rule was even valid fifty years ago.
In fact, like is more euphonic than as if when the complementary clauses are elided.
He drives like crazy. [Typical usage.]
He drives as if crazy. [Not used as much.]
(It almost needs not mentioning that as if and like are not otherwise interchangeable because of obvious counterexamples such as when like is a verb, or when it applies to a noun phrase: "I like Baroque composers, { like | *as if } Bach and Vivaldi.")
Xsounds like + {description or imitation of soundX}. X` sounds as if + {description of cause of soundX}. They can be the same, if the sound is iconic for its cause (like an explosion).