dplug is a framework for building audio plugins that do realtime signal processing, and its creator has produced several well-sold plugins under the "Auburn Sounds" studio name.

https://github.com/AuburnSounds/Dplug

D by definition meets the FFmpeg's criteria because it's also a C compiler. Because of that I never wondered how D performs in the benchmarks, as I know for sure that it can give me the performance of C where I need it.

Sociomantic (bought by Dunhumby, now defunct IIRC) had a realtime advertisement business built in D.

Weka have a realtime distributed filesystem written in D, used for ML/HPC workloads.