Whoever's song it is (lyric-wise) is on vocals, with a few exceptions

No Need To Be Downhearted (Part 1)
Thomas: lead vocal

Life In The Backseat
Thomas: lead vocal in verses/Alex: lead vocal in chorus/Thomas: chorus harmony

Woken By A Kiss
Thomas: lead and harmony vocals

If That's The Case, Then I Don't Know
Thomas: lead vocal in verses and chorus/Alex: chorus harmony

Shore Song
Thomas: lead vocal throughout/Alex: chorus harmony

Misunderstanding
Thomas: lead vocal in verses/Alex: counter vocal in verse/Alex: lead vocal in chorus/Thomas: chorus harmony

Secrets
Thomas: lead and harmony vocals

Cold World
Thomas: lead vocal throughout/Alex: chorus harmony

Have You Ever Felt Like It's Too Late
Thomas: lead vocal throughout/Alex: chorus harmony

Come Back Inside
Alex: lead vocal throughout/Thomas: chorus harmony

Appropriate Ending
Thomas: lead vocal throughout/Alex: chorus harmony

No Need To Be Downhearted (Part 2)
Thomas: lead vocal