I have a Canadian friend who sometimes helps me improve my English. A few days ago she sent me a list of some words (nouns) which the plural form is the same of the singular. One of these words was "music". I said to her that I didn't know that "music" don't need the "s" at the end to function as a plural word, and that it sounds wrong for me to say something like this: "I played two music". Actually, I was suggesting that, for me, "musics" sounds more "correctly". Then, she said that "I played two music" is indeed wrong and the correct is: "I played two pieces of music" or "I played two musical pieces". Can you explain why, And give some more examples of ways to use the word "music" in the plural form?
Music is an uncountable noun in most senses: that is a word that refers to a group or an amount of something, or to some broad concept that there cannot be two of.
Music is an art or a human category of sounds; it is not a particular instance of something related to this art. For such an instance, you could use piece of music, song, number, movement, ballad, ouverture, etc., depending on what kind of instance you were thinking of.
The same applies to nouns like water: you can't say "I have two waters for you" (except in special circumstances), but rather "two gallons of water", or "two cups of water". Nor could you say "I have many loves for you", or "could you move a couple of sands closer to the beach". A few instances of common uncountable nouns:
- a piece of music
- a piece of information
- a piece of advice
- a grain/pinch of salt
- a piece of furniture
- a piece/scrap of paper
- a blade/patch of grass
- a piece/log of wood
As you see, specific kinds of instances of uncountable nouns can be indicated by different countable nouns, such as piece, grain, patch, etc. Some of these uncountable nouns can be countable if used in a different sense. Note that it is not always obvious from meaning whether a noun is countable or not: to some extent that is something that needs to be learnt word by word, alas.
A short answer: it is not right to say either one music or to say two musics. You can't use a number with it. It's okay to say I played music. This is because music is an "uncountable" noun, which Cerberus gives details about in his answer.
"Musics" can be used as a plural when referring to multiple disparate musical heritages (or at least, I've seen it used as such). For instance, imagine one is comparing the rhythms of Classical Gamelan with those of Western Classical (e.g. Mozart). In that case, I believe the following would be a correct usage: "one of the similarities between these two musics is as follows (blah blah blah)".