The QuickTime that I know of is a system for playing video and audio files, all digital. I underatand that the system did once include some functionality for playing midi files, but this has now been removed. So QT would have included some sort of virtual synth, maybe a separate package included within QT, in order to render the midi data and create the actual sounds.
If such a virtual synth is still there, your new setup may be able to address it by name, but I doubt this will be 'QuickTime', maybe you need to find out what this part of the package was called and link to it by name.
If this does not help, then you'll need to find a new virtual synth system to replace that included in QT, and install that. Then ensure that your new software links to the new system as midi device.
Geoff