FBReader uses Qt Multimedia, Qt Multimedia Widgets and Qt TextToSpeech, copyright The Qt Company Ltd. and other contributors, under the GNU Lesser General Public License version 3.
The FFmpeg backend uses FFmpeg libraries, copyright the FFmpeg developers, under the GNU Lesser General Public License version 2.1 or later and the additional licences listed below. The distributed builds use shared libraries without GPL-only or nonfree FFmpeg features.
The read-aloud sentence preparation is adapted from Android FBReader's SentenceExtractor, copyright (C) 2012 Hyperionics Technology LLC and (C) 2004–2026 FBReader.ORG Limited, licensed under Apache License 2.0. The desktop adaptation uses C++/Qt, keeps original text positions and adds Unicode sentence endings.
These library licences do not require FBReader itself to be licensed under the GPL. Your rights to modify and replace the LGPL libraries, and to reverse engineer the application for debugging those modifications, are not restricted by the application's licence.
Upstream source projects: Qt Multimedia Qt TextToSpeech and FFmpeg.