Erm, this is catastrophic exegesis, shooting from the hip. - If read the lexicon carefully, you'll notice that Thayer's refers to a
specific usage of 'akouo' in Matthew 13:15, in a
specific context,
not all usages.
"For this people's heart is waxed gross, and their ears are dull of hearing, and their eyes they have closed; lest at any time they should see with their eyes, and hear with their ears, and should understand with their heart, and should be converted, and I should heal them." (Mt. 13:15)
There's about 20 different usages of 'akouo', ranging from "to be endowed with the faculty of hearing (not deaf)" to "to perceive in the soul the inward communication of God" (See Thayer's,
Genesis 1:1 (KJV)).
Now,
While it's true that the Lord said "Who hath ears to hear, let him hear." - 'Hearing' ≠ 'Understanding'.
In Matthew 13,
When the disciples asked, "Why speakest thou unto them in parables?"
"He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given. ... Therefore speak I to them in parables: because they seeing see not; and hearing they hear not, neither do they understand." (11-13)
Proof that 'Hearing' (akouo) ≠ 'Understanding'.
Think about this: Most people have 'heard' another language being spoken, but how many will have 'understood' it ? - In the same way, you're drawing false equivalencies and jumping to conclusions.
Hence, in 1 Cor. 14:2 (another usage of 'akouo'), Paul writes "For he that speaketh in an
unknowntongue speaketh not unto men, but unto God: for
no man understandeth him; howbeit in the spirit he speaketh mysteries."
See the Ancient Greek usage of 'akouo' here, for comparison,
ἀκούω - Wiktionary