First, make sure the DLC is installed - this should happen automatically.
Then check your Steam folder for the following directory:
Steam\steamapps\common\SlimeRancher\Slime Rancher Original Soundtrack
If you are unsure as to if the DLC has been installed you can double check by:
- Open your Steam Library
- Right click on Slime Rancher and select Properties
- Select the DLC tab and make sure the box next to the soundtrack is checked