Just because you can hear sound after dropping a MIDI file onto Renoise, it doesn’t necessarily mean the sound is created by Renoise.
Any of the following things may be true:
MIDI - Instrument are routed to a MIDI synth, such as the built-in one in Windows, OSX etc - or external hardware (you didn’t mention this)
Plugin - Instrument are using a plugin to generate sound
Sample - Instrument are native Renoise sample-based ones
You can check it yourself - select one of the instruments in the song (instrument list) and check what the MIDI/Plugin/Sample tabs reveal?
If you’re using plugins, you can render them to samples and thus, keep them bound to the song itself, and edit them in any way you like.