Someone once said to me “you can’t really hear what’s beautiful, since everyone’s perception of beauty is so personal”. I think that could certainly be true, but I also think that the average person would say that a donkey bray is not as “pretty” as the sound of a songbird. I think that people know when something is abrasive or harsh it isn’t as nice to hear as when it is soothing and smooth. If you make the criteria too intellectual or abstract, you can get quite confused. If you keep it simple, then it isn’t rocket science.
Human beings react to emotion. We like what feels good. We don’t like what feels bad. There are more emotions that are unpleasant (fear, anger, sadness) than pleasant (happiness, peace). The stronger the emotion being expressed, the more likely it is to get an emotional response from someone else. Could be the same feeling (rapport) could be the opposite (discord). Sounds that are happy, joyful, contented, loving, sweet, grateful, satisfied, or peaceful, connected to lyrics that express those sentiments, would be expected to be more or less pleasant in vocal quality. We all know that a lullaby sounds soothing and we all know that it sounds different than the barking calls of an Army drill sargeant. Sounds that are angry, sorrowful, fearful, hopeless, painful, distressed or agitated, when also connected to lyrics of the same intention, are probably not going to sound pleasant, and could actually sound very unpleasant. They could, however, also be very communicative, on either side of the argument.
So one of the primary ingredients of “real” singing is that it is always connected to authentic emotional expression. It is not machine-like. Of course, people can choose to sing in a mechanical manner, imitating a computer, but it would be unlikely that such singing would appeal to a mass audience. To an educated elite audience, maybe, but to the average person, probably not. That’s one of the reasons the 20th Century classical music did not find a large and new audience. People can only relate to it if they have music education and sometimes not even then.
The other driver is that the sound is freely produced, which means that the emotional expression is unlabored. Even the “unhappy” emotions are delivered without a struggle. Feeling what you feel as you feel it, while you are singing, and sounding unlabored while you sing, will resonate with others as long as the musical ingredients are also there. They would include singing with pitch accuracy and good intonation, singing the words clearly, singing with control over volume, and in many cases, singing with vibrato, but not always.
Real singing is very direct. It hits you somewhere that you cannot forget. It can be small and sweet, as in a child or an untrained adult, or it can be big, powerful and highly trained, as in an operatic vocalist, or it could be anything in between. One absolute is that it is unique, memorable, and distinctive. Real singing allows the uniqueness of the individual voice, the intention of the lyrics as emotion and communication, and the musical components of melody, rhythm, tempo, key and accompaniment to meld into one cohesive whole that is more than the sum of its parts. Singing loudly may be impressive but that’s all it is. Singing something that sounds ugly, without having a reason for it to sound ugly, is just plain stupid and sad.