You want to simulation the effect of horns and waveguides on a speaker design? Check out the software from Randteam.de. It uses lumped element models for acoustics modeling. This is not a “plug-and-play” software where you simply insert your horn design and click “run”. It will need at least some training, but the results seem to match reality quiet well.
There is a free version that does not allow saving. The commercial version of ABEC2 that is still in work.