singable
Easy and pleasant for most people to sing along to.
Singable describes a song or melody that's easy and pleasant to sing. A singable tune has a melody you can remember after hearing it once or twice, with notes that don't jump around too wildly and words that fit naturally with the music.
Think about songs everyone knows at summer camp or in music class. “Happy Birthday” is highly singable because the melody flows smoothly and anyone can join in. Compare that to some opera arias or complex jazz pieces that might sound beautiful but require years of training to perform. Those aren't very singable for most singers.
Songwriters often aim to make their choruses especially singable because they want audiences singing along at concerts. A singable song usually stays within a comfortable range of notes (not too high or too low) and has a rhythm that matches how people naturally speak. The opposite would be a song that's technically impressive but too complicated or awkward for most people to sing comfortably.
When you find yourself humming a tune hours after hearing it, that's often because it was singable. The melody stuck in your mind because your brain could easily process and reproduce it.