It is now possible to read a midi file in the version 2.2.0 of the software. It is a real advance.
On the other hand, I would want to know if it is possible to change the tone of instruments affected by default.
Beforehand thank you for the information.
Audacity only provides simple cut / paste type editing of the MIDI track. There is not yet any facility to add or modify “Program Change” message (or any other message type).
Depending on the MIDI synth / sampler that you are using, you may be able to change the sounds associated with specific patch numbers. For example, if using Timidity++ or any other synth that supports “Soundfonts”, the sounds could be changed by loading a different Soundfont.