Idina Menzel's pronunciation?

I've been listening to her singing for ages, and I've never noticed this.

Upon re-listening to the recordings you cite, I can hear what you're talking about, but it's not really replacing or mispronouncing the sound. Linguistically speaking, 'n' and 'l' are very similar sounds. Likely the reason one comes out sounding like the other is because of diction, and the position her lips and tongue are in when she hits the note (singing requires a lot of mouth contortion!). It's likely not intentional, or even something she notices as she's doing it.

Singers often cheat a bit on their vowels and consonants to facilitate easier/more accurate vocal technique. If you'll notice, the two sounds are pronounced clearly most of the of time when she sings, even in those particular tracks. It's only when, due to the sounds or pitches required, her mouth is in a different shape that it comes out sounding a little different. To me, it still pretty clearly is an 'n' sound.

I of course have no idea if it's actually a mispronunciation issue she has, but having listened to hear sing and talk at length, I don't think it is. I'd be willing to bet within a reasonable certainty that it's just singer technicalities.