Q Light Controller+

Currently QLC+ only supports a single audio output, the destination of which is selected on the I/O audio tab, which is inadequate for many theatre productions; the BASS audio backend is used by a number of well-regarded playback systems, both paid-for and free, and although not open-source, it is free for open-source projects, and is supported on Windows, Linux and Mac, and does do all the right things for a theatre playback system, and would enable multiple simultaneous playback through multiple outputs. This would enable the destination to be selected on a per-playback basis, over-riding the default. BASS also has many other abilities, so it is a foundational piece of improved audio capabilities. https://www.un4seen.com/

There is a global audio output device and a per-function one.
Each audio file can be played on a different audio device, so what you need is already there.
Otherwise, please describe a little bit better the theatre usage case; e.g. which audio device do you use?