Light jazz music is the easiest to perform not because of the melody but the freedom to express the message of the piece. It also calls for musician who has the passion to interpret the song or music in line with the vocal rendition matching the feel of the singer to create the magic of a song. 
It doesn't require you to be musically schooled to perform such a feat. It also needs the ear for sentiment inherent in each of us. The more sensitive you are, the better performer can you be. 
Well, music has to be learned first to know its nuances and to exact the melody according to what it intends to send. 
Everyone can interpret or play particular music or piece but what sets you apart is the difference of talent, meaning a gift given to you especially if you are a singer. And the musician who rules such perfection can enhance the vocal part for both to shine. Of course, the vocalist has to match the cadence of a true artist.

An example of such performance doesn't require you to raise your voice or reach high notes. The fluidity of carrying a tune is akin to a conversation where there's no need to raise voices but naturally expresses one's feeling. Even when joining a singing contest, it irritates the judges when you belt too much. They still long for someone who knows how to control his or her voice. What is singing for but control.