Hi! I’m just finishing the first release of libdolbyb, 1.0-rc3,
and if all is well 1.0 will be out ina few days.
Is anyone interested in seeing if it can be incorporated into audacity
so the millions can more easily de-dolbyb their archives of cassette recordings.
Until libdolbyb gets into the distros I guess its source will have to be packaged with
audacity, the same as it is for SoX_ng which has it as an effect.
Or should I file an issue and have a go? (I’m a bit busy on other things!)
I can’t speak for the developers but Audacity supports VST and Nyquist plug-ins. If you make a compatible plug-in, anybody can use it.
..I’m sure you know that Dolby noise reduction isn’t linear and it needs a calibration level. There’s no fixed or standard calibration between the analog tape levels and digital levels so that’s a BIG problem. 
…If you could get a Dolby calibration tape, which I haven’t seen in years, the person doing the digitizing could calibrate their tape machine to their audio interface to set the cal levels.
P.S.
It wasn’t hard to find at all! Dolby B Calibration Cassette
Yes. A tape deck knows the level on the tape but once it’s digitized it could be at any volume. dolbyb/libdolbyb has a “threshold gain” setting which is used to compensate for this and so far the recommended procedure is listening tests until a good result is obtained
With a calibration tape digitized at the same volume settings as when digitizing the actual tapes, it may be possible to get a right setting for all of them, though I’m not sure what the procedure would be.
I’m afraid I haven’t made VST/Nyquist/LADSPA plugins before, but that might be a better route, yes. There is also GitHub - prof-spock/SoX-Plugins: Reimplementation of the SoX Command-Line Audio Processor as DAW Plugins which reimplements SoX effects as VST3 plugins, so it may happen first there.
If an approximation will do, there’s AirWindows Dubly3 encoder/decoder …
It’s free & works in Audacity, VST2/VST3, (there’s even a 32-bit version).