Originally Posted by: FlyingJbird
I can only do this when a modulation occurs... now: do you mean I can only do this when the melody has to be changed to fit the chord?
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The melody doesn't necessarily have to reveal the modulation. But it can. Further ...

Originally Posted by: FlyingJbird
Or, would a modulation also occur if a chord being played has a note that does not fit in the given key (in this case, key of A minor)?
[/quote]
This can be evidence of modulation, when a note(s) present in the music is/are out of the home key (the key that the rest of the music is in). But, odd notes are not enough.

Consider a chromatic passing tone. Blues players do this all the time, put the flat five in a minor pentatonic scale over a 12 bar blues. You can have a simple C (I), F (IV), G (V) chord progression and play a C chromatic scale over it.

In both cases you only have accidentals, chromatic passing tones.

In order to have a modulation, you have to have notes that are out of key that suggest a change of key center through harmonic function.

Originally Posted by: FlyingJbird
Okay, so are you saying D dorian fits because it works with the chords?
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Yes, and more, because it fits the harmonic function of the chord progression. Dorian suggests the ii chord of a major key; ii chords are sub-dominant.

A min (vi) intermediate or substitute tonic
D min (ii) sub dominant
G7 (V) dominant
C (I) tonic

Originally Posted by: FlyingJbird
Isn't that the same as an A minor with emphasis on the D?
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Yes, but you are only thinking in terms of modes and notes. You need to expand that, build on that in order to see and hear harmonic functions.

Originally Posted by: FlyingJbird
Are there any other scales that would sound nice against that chord set?
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That is only a question you can answer. :) I suggest trying all the possibilities and listen for what you like. Remember to keep what you like, and avoid what you don't. And remember, how you use or play any given scale can make it sound completely different. Emphasize or de-emphasize chord tones!
[QUOTE=FlyingJbird]
Okay, I get how G7 implies an A minor - because G7 is the 7th of A, and the 7th of a minor scale is a dominant chord. It's the exact same with the B half-diminished in the first chord set.

Kind of. Again we are looking at harmonic functions. G7 implies C major:

A min (vi) intermediate or substitute tonic
D min (ii) sub dominant
G7 (V) dominant
C (I) tonic

This is clearly C major. I would say the song is in A minor only when there is an E7 (V) resolving to an A minor (i). Like in the first 2 measures:

Bmin75b (ii) sub dominant
E7b9 (V) dominant
Amin (i) tonic
[QUOTE=FlyingJbird]
Yeah, I noticed that actually - that Bb is played on the second "verse" of the melody. So this is definitely an instance in which A phrygian scale (d minor) works.

Sure, but more importantly, look at the harmonic functions involved. From m.17, we start in the key of A minor.

Amin (i of Amin) / Bmin7b5 (ii of A min) - E7b9 (V of Amin) / Amin (i of Amin) / Bmin7b5 (ii of Amin) - E7b9 (V of A min) /

Interrupted resolution! Now we switch to the key of D minor!

Emin7b5 (ii of Dmin) / A7b9 (V of Dmin) / Dmin (i of Dmin) / Dmin (i of Dmin)

Now we can look at this next line two different ways. Either as in D minor for 2 measures, then switch the A minor.

D min (i of Dmin) - Dmin7/C (i of Dmin) / Bmin7b5 (ii of A) - E7b9 (V of A) /

Or as an immediate switch back to A minor.

D min (iv of Amin) - Dmin7/C (iv of Amin) / Bmin7b5 (ii of A) - E7b9 (V of A) / A min (i of Amin)
[QUOTE=FlyingJbird]
The one thing I'm not sure of is A7b9 being the V of the Dminor scale - or rather, the notation. A7 is a major chord with a minor seventh. So where does that major third come from, since both Amin and Dmin have the A chord as a minor! Unless that's intentional dissonance to make the Dmin7 resolve better.

The music changes to use the C# as the leading tone of D minor scale! We are now in D harmonic minor. This is the evidence that we have a modulation to the key of D minor. A7 is a dominant chord.
[QUOTE=FlyingJbird]
So leading up to the chord, it's slightly dissonant, but it makes the chord resolve much better. And then, just as the chord resolves, I want to jump to a new scale?? So basically keep teasing baby, and keep that candy juuuuuust out of reach. :P

Precisely. Suggest or imply the chord change that is coming up, and then change to suggest or imply the next one. This is why some jazz (especially bebop) can sound so "weird" or "outside". It is always implying what is coming next, even if or especially if that clashes with what is currently happening.
[QUOTE=FlyingJbird]
How does D dorian go over that Amin7-Dmin7-G7 chord set, though?

It only seems mystifying because you aren't looking at the harmonic goal! You left out the end goal of those chords, the C major:

A min (vi) intermediate or substitute tonic
D min (ii) sub dominant
G7 (V) dominant
C (I) tonic

Ergo:

A min (vi) intermediate or substitute tonic - A aeolian
D min (ii) sub dominant - D dorian
G7 (V) dominant - G mixolydian
C (I) tonic - C ionian

Which is all just C major scale notes. However, you need to use them to emphasize the chords that are happening at the time in a way that matches your sense of melody.