First off, please make sure to have app version 4.0.3.
Once you are in, select any track, and tap on Start Session.
Then, tap on the Background sounds Off option listed there
Now move the slider to the left until you see the Edit Background set option and select it.
At the new screen, you will see the list of Sounds
Here, you can choose up to 5 to be used with your tracks at any moment, do so, and save them.
They will appear now in your tracks for you to use.
If you need to update them, just follow the same steps.