In this paper, we aim to contribute to the understanding of how readers experience immersion in digital reading experiences, more specifically with digital reading supported by (3D-)audio tracks. We formulate user and content requirements for implementing (3D-)audio soundtracks for readers in a digital reading application. The main research question addressed in this paper is: (how) can audio aid the immersion of readers in digital fiction stories? To answer this question, three online focus group discussions were organised in Belgium and Germany. As part of the set-up of the Horizon Europe project Möbius, 18 participants tested different 3D-audio tracks while reading via the Thorium Reader application. The results first address how participants define immersion, and how the role of audio in immersion can become paradoxical. Then, the paper presents a detailed evaluation of the factors en- or dis- abling immersion for the specific 3D-audio tracks, and how these insights can be implemented in reading apps via user and content requirements.