"Say what sounds right."
Bad advice has an annoying habit of sounding like good advice, and this little phrase really is something of a wolf in sheep's clothing. It's definitely appealling when someone points out that that's what we do in our own language.
I wouldn't argue that my end-goal isn't to be able to simply say what sounds right, but I just can't see how that end-goal affects my learning path: nothing will never "sound right" until I've learnt it, so how can I learn by what "sounds right"?
The consequences of this are not insubstantial, because if I've not learned it yet, what's going to sound right to me? What sounds right is something that I have learned, but this will be out of context.
Take for example the verb "start" in English.
You can "start something".
You can "start doing something".
You can "start with something".
You can "start by doing something".
If you learn only two of these, then only those two will "sound right".
Saying "what sounds right" traps you into what you know and stops you expanding your language. What you need to do is stop and think, and use the appropriate form, even if it isn't familiar enough to "sound right" yet.
Here's another example.
French has a feature called "liaison" -- certain final consonants are silent but reappear when followed by a vowel, but only if the two words are tied together syntactically.
In the word "vous", the S is normal silent, but in the phrase "vous allez" (you go), it has a /z/ sound.
Now, the past participle of to go is "allé", which is pronounced identically to "allez", so when you ask "êtes-vous allé" (have you gone), if you go by what "sounds right", you might pronounce that /z/. But in this question "vous" and "allé" are not syntactically bound together and liaison should not occur.
The idea of "what sounds right" reaches a very messy conclusion in Scottish Gaelic. A single syllable consisting of a schwa before a noun can be one of three things: "the", "his" or "her". "his" always causes initial lenition (soft mutation of the first consonant) of the following noun, "her" never does. As "the", this form can occur before masculine and feminine nouns in certain cases, and causes initial lenition, and only with certain letters.
Many teachers suggest that you learn noun gender by "what sounds right", by agreement with the article, but your ear will be exposed to the various case-inflected forms and possessives, so what sounds right might not be "the boy" at all, but "her boy" or "his boy".