Lol I literally have individual spotify playlists for each of my projects for this very thing
Here's a couple for "Project: Achilles".
Dutch: I have two songs that I use for Dutch, but the one that fits him the best is "God's Gonna Cut You Down" by Johnny Cash. I think it shows his main motivation pretty well.
Alex: Alex hasn't really been introduced yet (he'll be in my next update on Monday, though), but his theme song is "Waving Through a Window" from Dear Evan Hansen. He's a really shy, really nervous kid, who's trying to gather the courage to be more like his older brother Wyatt, whom he idolizes.
Col. Aldridge: Col. Aldridge's theme song is "Devil's Right Hand" by Johnny Cash. I don't have any real specific reason for picking it for him - just kind of sounds like him, I guess.
Franz Teudt: For Teudt, I picked "Mvt. 4: Marsch" from Hindemith's Symphonic Metamorphosis. Feels like him, and I think it's something he would jam out to. As much as you can jam out to classical music lol.
And here's a couple links to read "Project: Achilles"