Spontaneity in music

I'm not talking about improvisation, at least not right now. I'm talking about spontaneous performances of composed music. One example I have come up with is a way for people to suddenly break into choral music that can be sung with any combination of the soprano, alto, tenor, and bass parts. The basic form is strophic (e.g. using the same music for each verse) with one modification: the first verse is sung in unison. This allows everyone to have access to the melody, which is crucial, and the reason for this will become apparent in the following scenario:

In this scenario, anybody who wants to participate wears a button with an S, A, T, or B, according to that person's vocal range. This button is worn anytime the performer is free and willing to strike up a song with someone else, most likely with strangers. A woman wearing an A comes across a man wearing a T. They agree to sing something. They pick a tune with which they are both familiar and, as is common practice in this system, sing the first verse in unison. Then, the alto takes the melody and the tenor sings his part, which is written in such a way that it sounds decent when sung with the melody alone. Then they switch, the tenor taking the melody and the alto singing a similarly fitting line. At the third verse, a man wearing a B button approaches, and the three sing together. After finishing this first tune, they sing a second, and then they continue doing whatever they were doing before. All this without a soprano, the part that typically has the melody.

If all four parts were familiar with the melody and at least one harmonization, then any combination of people could sing. Any part could be included or left out. In order to facilitate people of varying skill levels, much of the music would have to be fairly simple with limited ranges. That makes it no less fun, however, to surprise people on the sidewalk or at the mall by singing a few independently rehearsed songs.

This type of performance fits perfectly with the compositional philosophy that I'm trying to create for myself, which is centered on environmentalism but includes performing music for the performer's sake. Today, performances are generally regarded as an event that includes an audience that attends specifically to see the performance. Why can't music be performed just because the musicians feel like doing it? If it is done in public, then the audience consists of anyone who happens to be around, and people may stop and listen if they choose. Most important is that this type of choral performance is a zero-emissions performance: it happens under circumstances where the performance site requires no special lighting, sound systems, or physical preparation. Additionally, because the audience consists of people who are already at the performance site for completely different reasons, no fuel is consumed by people seeking music. The only resources consumed would be by those learning the music, most likely in the form of paper or electricity.

The challenge to composers in this medium, then, is to write and arrange music that it can be sung by any combination of the vocal parts. I believe that this can be done, and I hope to see something of this nature develop because it provides an outlet for people who wish to sing, and it also brings free entertainment to anyone who cares to listen—or chime in.