Just about all Irish consonants come in two versions, usually called "broad" and "slender". (I've also sometimes seen them called "velarised" and "palatalised", respectively.)
But since the Roman alphabet doesn't have enough consonant letters to make broad and slender consonants separately, the vowels do that job.
So some vowel letters are sometimes there not because they are pronounced, but to mark the consonant that they are standing next to as broad or slender.
a o u stand next to a broad consonant, e i next to a slender one.
That explans the different sounds you heard: léann and leabhar have a slender L, while labhraíonn has a broad L.
Slender consonants sometimes sound as if they have a "y" sound in them.
I saw someone explain it in an earlier lesson. What duolingo should have written in the notes is that with verbs ending in "-igh," you should remove the "-igh" for both 1 and 2 syllable words. Since "léigh" is a 1 syllable word with a slender vowel, you use the 1st set of conjugations, which is why it's conjugated as "léann" and "léim." And the reason why it's "léann" and not "léeann" is because you remove the extra "e." I was confused by this too until I saw someone else explain it.