Is it true that all music must stick to the scales for that key, or can this vary?

Answer 1

No, and the music is usually more interesting if it doesn't. This is sometimes called using "blue notes". It can be done on purpose to deliberately throw off the listener - it might sound like a "sour" note at first, but if its repeated then it becomes part of the pattern. I frequently do what I call "chromatic scales layered over the pentatonic". The most interesting music frequently "breaks the rules". Don't go too nuts with this concept or it will sound awful, but if that is your goal, then go for it.

Answer 2

The keys themselves can vary, and usually do. There are also many variations of the same scale that sound the same. EX: C Major-c minor
are the same basic scale but have different sharps and flats.
In other words, no, music may have different keys and scales. In addition, music may also have accidentals where a note that is usually sharp may become natural, or vice versa.

Answer 3

Music does not have to be in any particular key - atonal music for example. A piece of music is only said to be in a particular key if it follows the range of notes in that key and resolves itself to the 'tonic' of that key, i.e. the route chord of the key. e.g. most pieces in the key of G major would end on a G major chord.
Composers tend to compose according to the 'rules' of harmony because that's what we find to be pleasant sounding. If your ear cannot discern the key, then you would think that everything was out of tune. The key fixes a reference point for your ear. Try whistling a well known tune and deliberately get the last note wrong and you'll see what I mean.

Answer 4

The theremin plays a continuous range of tones (Beach Boy's 'Good Vibrations'). The sitar is another good example.

Answer 5

You should only listen to more complex bands to hear how often it can change.

Listen to a band like Dream Theater.

Answer 6

it can vary