I am trying to export an audio file of a wind band composition. I extracted the Euphonium part to make a treble clef version of the part. Now, when I export the audio file as VST, the only instrument on the playback is the euphonium. I have tried every trick that I know to change it to reproduce all the instruments.

I can export a MIDI version, but it lacks the quality in sound of the VST. What do I need to do?