To select session audio on the Mindbloom app:

Select the Home tab by tapping the house icon on the far left bottom corner

From the session card, under Audio, tap Preview to listen to the track or Change to change the track

The previewed session audio track will appear onscreen, and you'll be able to exit the preview when complete

To change the session audio track, you'll be taken to the Audio Library

In the Audio Library, use the filters to select a session audio track