What triggers the Avenger music change?

Question

What triggers the change of music on the Avenger?

In XCOM 2 (and XCOM 1 as well), the story follows a Three Act formula denoted by a different music track playing during each act while on The Avenger. The tracks in XCOM 2 are titled Stage One, Lazarus (for the second stage), and Stage Three. However, as the differences between the tracks are relatively subtle, it can be difficult to notice exactly when they change.

What events trigger the switch between acts, and consequently, between music tracks?

Answer 1

From the modding code, this seems to be the relevant code. This will contain spoilers!

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Chapter 1 will play until you build the Shadow Chamber Chapter 2 will play from building the Shadow Chamber until killing (I presume) the incomplete avatar during one of your missions Chapter 3 will play from that point forward.

function PlayBaseViewMusic()
{
    SetSwitch('StrategyScreen', 'Avenger');

    if( class'XComGameState_HeadquartersXCom'.static.IsObjectiveCompleted('T1_M6_KillAvatar') )
    {
        SetSwitch('HQChapter', 'Chapter03');
    }
    else if( class'XComGameState_HeadquartersXCom'.static.IsObjectiveCompleted('T3_M2_BuildShadowChamber') )
    {
        SetSwitch('HQChapter', 'Chapter02');
    }
    else
    {
        SetSwitch('HQChapter', 'Chapter01');
    }
}