who have more correct/clear pronounciation: Avril or, for example, Tarja Turunen (ex-Nightwish). If you never heard Tarja I have many another examples: Cristina Scabbia (Lacuna Coil), Amy Lee (Evanescence), Sharon den Adel (Within Temptation) and so on.
the only one of these I know of is Amy Lee and she pronounces very well on the album Fallen. These are pop/rock "artists" so the pronunciation can vary from good to unintelligible. Get country or folk if you want to hear words sung clearly.