Rememebere tehe vowelese!
Retina, 48 bytes

i`(?&lt;=([aeiou]).*?[^\W\d_aeiou])(?![aeiou])
$l$1


Try it online! Explanation: The lookahead searches for a point not followed by a vowel, while the lookbehind searches for an immediately preceding consonant and a previous vowel, which is then inserted in lower case.