There is absolutely an ancient community function aspect to music. And a different consideration: much of music in every respect is geared to social appreciation. But consider that music itself is always a solo-individual creation. My guess in sum is that most people allow their pleasure in music to be provided by others who are good at it.

Prayer is about a relationship, the person(s) praying and God. Then praying alone in a bedroom or cell is a social thing. Thus it would be natural to chant but still people often speak rather than sing at those times. Why is this?

My guess is that private singing for most folks is a form of work the rewards for which aren't to them worth the effort - and likely they, those people, even forget to think about it. But it's time for you to have some ideas of your own.

What I do not get is:
Why is music so much about performance. I find that playing music or singing when alone is really great and important to me. I guess being alone is about practising (even if we do not think about music). What do you think?

Just some thoughts, but do you think that you need to consider the fact that music is listened to more often in groups? Concerts, places of worship, shopping malls, etc.? In order to even listen to music by yourself, you have to first be by yourself, which for most people means intentionally removing yourself from a group, whether it’s family members, friends, or the public in general. We are seldom alone.

That being said, I am the type of person that only sings alone, mainly because I’m insecure about my voice, but I would wager that there are others like me as well.

I love to sing, and I'm told that I'm quite good at it.

I've almost always got music playing in my mind, and if I'm in a good mood and by myself and not doing anything else linguistic (which, these days, is a rare confluence of conditions, limited only to solitary hikes on particularly good days) I love to sing by myself.

I used to sing on my own in the presence of other people when I was young and socially inept, but I received lots of negative feedback about that, so my social etiquette routines (that I purposefully trained into myself as a young adult trying to learn to navigate the social world better, but now have become subconscious habit) now have me usually quiet in the presence of other people; if I'm hiking alone and pass another person, I automatically shut up.

I absolutely love social occasions to sing though, like a karaoke night. And even more when other people join in. One of the things I loves the most about my old RHPS cast was the tendency of the whole group to spontaneously burst into song. (On one occasion at a restaurant after a show, a waiter walked by humming "Lollipop", one cast member made the "POP" sound on the appropriate cue of the waiter's hum, and the entire cast started singing in unison, spontaneously. On another occasion, someone walked through the dressing room humming one line of "Summer Loving", prompting another to sing the next line, which then evolved into a full-on ensemble performance of the song there in the dressing room, again completely spontaneously).

I think I had a point when I started writing this, but now I've forgotten it.

Singing can be a social thing, but it can also be an alone thing. Music is about relationships of sound - so I’d say that singing is more of a relationship thing, whether you do it alone or with others.

I have a tendency to sing alone in my car, or else with others who I know share my appreciation of music and song. I think often when we sing alone, it’s for company or reassurance - more effective (and less crazy) than talking to yourself, anyway.

Lately I’ve been enjoying the challenge of picking out a harmony when I sing along to music. There’s something about a collaboration of voices that raises the spirits and inspires a collaboration of effort and experience - like working or worshipping or concerts or karaoke or those sing-along ABBA and The Sound of Music shows...

If you like it, then you like it. I sing alone. I suppose in the long run singing in from of and or with others is more important to me, but we get to value what we want. Singing in the car and the shower are both cliches because a lot of people do that and usually alone.

There's music and there are words; when these two come together, we get singing. Music is there to evoke emotions and words there to convey a message. A song is then an emotionally charged message.

Since communicating with yourself is as real and as vital as communicating with others, singing may take the form of solitary self-reflection and also an opportunity to spread the word about something one thinks is of value.

I understand that it is less common to sing when being alone than when being with other people. Many parents sing for their kids but not when being alone. Monks sing in the church but not when being alone in their cells. People who sing work songs never do it alone in most cases. Why is it that singing when being alone is not that common? Is singing really only a social thing? — musicpianoaccordion

Do you have evidence to support the claim that all people of all cultures in all times "don't sing" in solitude?

I sometimes read or talk aloud when no one else is around. As I'm getting older, I do it more often, for longer, and more often in full voice. Why didn't I do this when I was younger? I think I was raised to consider it inappropriate or foolish. I've come to consider that attitude wrongheaded.

You've heard of people singing in the shower? I presume that like me, those people are most often alone in the shower while they're singing, and that they often sing louder in the shower when no one else is home. Why do you suppose that is?

From time to time I also sing, hum, or whistle when I'm puttering around and no one else is home. Usually rather quietly. I used to sing fairly loudly while walking down the street, whether or not anyone else was around. Sometimes I've chanted during meditation when alone.

Surely I'm not the only one.

I've almost always got music playing in my mind, and if I'm in a good mood and by myself and not doing anything else linguistic (which, these days, is a rare confluence of conditions, limited only to solitary hikes on particularly good days) I love to sing by myself. — Pfhorrest

Same here. Even when I'm not alone, I sometimes start singing to myself at a low volume. It annoys my wife.

I don't know how many people are like us, but I've always assumed we were pretty common. Maybe I was wrong.