We've recently teamed up with colleagues from SWR Online to implement alternative audio tracks in the ARD Player framework which is using HLS for adaptive streaming. This allows ARD to distribute alternative audio versions such as "Dialog+" for better speech intelligibility or a binaural version for 3D audio on headphones in an efficient manner without additional bandwidth. The feature is included in version 5.4.0 of the ARD Player and already rolled out in the ARD Mediathek. You'll find more information in our Blog.
Please note that on some iOS devices the track selection works currently only in fullscreen mode!