Let's talk vowels. AHHHH!
Did you know most kids are amazing vowel-producers? It's those pesky consonant sounds that are often the ones that trip them up. Think about it: we all know a kid or several who have trouble with making a /k/ or an /s/ sound correctly. But how many do you know who can't make an "ee"? Who say "tuh" instead of "toe"? Not many. Vowel errors are rare in typically developing children, and even in most children with your more run-of-the-mill speech delays and disorders.
The thing with vowel errors is this: they make it super hard to understand a child's speech. And when we hear these errors, we know something pretty unusual is going on for the child in question. So if you're concerned about a child's speech clarity, listen to which sounds they are having trouble with. If it's those vowel sounds (the "ah", "ee", "oh", "ooh", "ow", "aye", "ey", etc.), this will have a huge impact on whether they are understood by others and is often an indicator that they will need speech therapy to help improve! Early and intensive support can make a huge difference.