Since the game and the soundtrack were designed together, Von Hertzog and Saturday Development have decided to include a free copy of the soundtrack with every game purchased on Steam.
After you have installed Neon Aileron, you will find the full soundtrack in the Soundtrack folder in the location the game is installed. Let us know if you have any issues finding it.
The soundtrack will also be available on its own on Von Hertzog's Bandcamp page: https://vonhertzog.bandcamp.com/