Hi Sam,
something must have gone wrong with your initial installation. MIR 3D was made to simply replace legacy MIR Pro. In fact we spent quite some time during its development to make sure that "old" projects open flawlessly and sound indistinguishable im MIR 3D. Personally, I switch between "old" and "new" MIR quite often*) for various reasons, and I find that it simply works. 😊
However, if you you want to make really, really sure that everything works as expected, then please uninstall legacy MIR Pro before installing MIR 3D. It's not necessary to delete the legacy RoomPacks, though - MIR 3D relies on its own RoomPack format. Make sure that you have installed those, too, of course! BTW - Vienna Assistent will take care for these tasks.
As a last step, create a new project in your host, instantiate MIR 3D and make sure that all Venues load as expected. From then on you should have smooth sailing. 😉
... if you _still_ encounter problems, please get in contact with support@vsl.co.at - VSL's support team will take care for the issue.
*) EDIT: Re-reading my message above it occurred to me that it is necessary to point out that projects created with legacy MIR Pro are upwards-compatible, but projects created with MIR 3D are _not_ downwards-compatible. IOW: Projects created with MIR 3D will _not_ open in legacy MIR Pro!
Kind regards,