Is it "I Hear Music" by Billie Lane? Not exactly a one hit wonder, died in 1959, no lyrics of "let me in". I must be right!
I hear music
Mighty fine music
The murmur of a morning breeze up there
The rattle of the milkman on the stair
Sure that's music
Mighty fine music
The singing of a sparrow in the sky
The perking of the coffee right near by
That's my favorite melody
You my angel, phoning me
I hear music
Mighty fine music
And anytime I think my world is wrong
I get me out of bed and sing this song