Soloist
A 'one word noun' is a noun that is a single word for a person, place, or thing.
The likely word is "individual" (a single person, or a single item).
You know if a song is a single if only one artist is singing in it.
They would be called a soloist.
Solo.
No, the word "someone" is not a compound word. It is a single, indivisible word that combines the words "some" and "one" to refer to an unspecified person.
Yes, the word 'one' is an indefinite pronoun when taking the place of an unidentified person. Example: How long does one work here before being offered full time. The word 'one' is also a noun and an adjective. The noun 'one' is a word for a number; a single thing; a dollar bill; a word for a thing. The adjective 'one' describes a noun as a single person or thing.
they have the same singing voice it is one person.
A "solo".
There are 2 syllables in the word singing. Sing-ing.
one person singing by him/her self
Syllabic is when a composer uses a different note per syllable of a word. Mellismatic (I think that is how you spell it) is when a syllable of a word is sung to more than one note. I hope this is what you meant and that this answers your question.