In the case of Azaria, I can assure you that's how he was directed to sing Moe's line. Hank has other characters he sings better for (Apu, for instance) but he has exhibited no real issues with his line delivery unless he's directed to read a line a certain way.
Kavner is a different story. Julie's Marge has been getting rougher over the years. At first, it largely varied based on the direction but now her delivery is showing real weakness even when an episode is otherwise directed well. I think this is a case of where we need to temper our expectations. She's 65. She can't defy age. She's getting paid substantially more than the person that's directing her in the booth. They're not going to make her read a line back 20 times until they get the one take the engineer can salvage.
That said, even though she was still rough throughout Halloween of Horror, I did really enjoy her read of "Who wants to be a yoda? YOU want to be a yoda!" when she was talking to Santa's Little Helper in the opening scene. That was very cute.
For those of you who enjoyed Halloween of Horror and want to keep tabs on the rest of the season, be sure to check out the
Season 27 OT.