To install, go to Library > Music and click on the + . Add the folder where Light Apprentice is installed (usually C:\Program Files (x86)\Steam\steamapps\common). Click on Scan. It should all be working if it looks like this:

Track list:
01 Toughest Journey
02 What's in Here_
03 Battle Theme
04 Explorers' Demise
05 Game Over
06 Stuff Gets Serious
07 Legend of the Five Apprentices
08 Nate's Theme
09 World of Ethenia
10 The Longest Journey
11 Boss Theme
12 Victory!
13 Orcs' Theme
14 It'll be OK
15 Mya's Theme
16 Brainwash Concert
17 From the Darkness
18 Watch Out
19 Yrohsian Empire