I ask this for a videogame soundtrack but I guess the answer would apply the same to movie soundtracks or music albums.

Ideally, should the keys of all the songs be related or gradated accordingly?

For example, if the first song is in C, the next one can be in C, Cm, Am, G or F but cannot be Abm.

Or it doesn't matter at all?

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    Why do all the "songs" even have to be "in a key?". A lot of the music on movie soundtracks certainly isn't! Your "only follow C with C, Cm, Am, G or F" is a good recipe for "writing elevator music", not something you might hope people will remember for more than 5 minutes. – user19146 Feb 27 '17 at 4:36

If the songs are distinct (i.e., don't overlap), then it doesn't matter at all. You can end a song in one key, and then pick up a new theme/song in a different key. There are no rules that you need to follow.

If the songs do need to flow into each other, it will require a little more thought. You probably want some sort of modulation between the two keys. You can pretty much modulate from any key into any other key. So you can still choose any pair of keys you like. The audible/emotional effect of the various modulations differs, so you probably want to choose something that will complement the intended mood change between the two songs. Or lack of mood change. You have a lot of options to choose from.

If you want to learn more, try searching using the term 'modulation', or perhaps 'key change'. Our tag might be a good starting place:

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If you are composing for a continuous performance, such as a musical, or putting together tracks for an album meant to be listened to all the way through in order, then it is a good idea to vary key or scale after some time so your listeners don't get tired of hearing the same thing.

In recording projects I generally try to avoid putting more than three pieces together with the same tonic (i.e. all in D). Changing rhythmic variation and scale (major vs. minor) can help, but I find too much "sameness" after about three pieces.

For musicals I find that the compositions will often need to be altered to a key that suits the principal singer(s) range, changing it up anyway. Otherwise I make the attempt to have some variation between songs. It also depends on how much time there is in between songs, and what incidental music is put in between. The incidental music can borrow the themes from the songs to change the feel or emotion (put the love theme in minor when the tragedy happens, increase the tempo of the Hero's theme in the action sequence).

Going to related keys seems to me to have a better flow. Some easy transitions I've found are:

  • Going to the dominant of your previous key (up a 5th) works well, especially in continuous music transitions.

  • Switching to parallel minor to get to the minor's relative major (such as going from G major to G minor then transitioning to Bb major)

  • Changing to the relative minor.

Chromatic and Step-wise transitions will also work, but if there is too much time in-between the change you can loose the sense of the change and it will sound like the previous key, especially in chromatic changes. Transitioning downward (such as C to B or Bb) can make the music feel like it is dragging, or cue sadder emotions.

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IMO, it doesn't matter that much whether the keys of themes in a video game are related or completely different. I do not recommend keeping all the themes in a video game in the same key, though. A basic example is that your victory theme should probably be in a major key (to encourage people to win) and your "game over" theme should probably be in a minor key (to discourage people from losing).

For video games, you may need to keep other conditions in mind when assigning keys to themes, such as the pitches that sound effects use. For example, I believe that Undertale's "Death by Glamour" is in E flat minor in-game but E minor in the official soundtrack because the soundtrack version may have been created first, but as this game footage video shows (especially at 6:53), "Death by Glamour" is the boss theme of a character who speaks in E flats.

Movies can get away with keeping more of their themes in the same key. For example, several of the most famous songs in the Pirates of the Caribbean: The Curse of the Black Pearl, such as "He's a Pirate" and Jack Sparrow's motif, are in D minor.

You can successfully get away with atonal music in any kind of soundtrack, though. Soundtrack music doesn't even need to be in a key at all!

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