For the instrument:
While you are playing this musical instrument, you can create harmless, illusory visual effects within a 5-foot-radius sphere centered on the instrument. If you are a bard, the radius increases to 15 feet. […] The magical effects have neither substance nor sound, and they are obviously illusory. The effects end when you stop playing.
And for the cantrip:
You create a sound or an image of an object within range that lasts for the duration. The illusion also ends if you dismiss it as an action or cast this spell again.
If you create a sound, its volume can range from a whisper to a scream. It can be your voice, someone else’s voice, a lion’s roar, a beating of drums, or any other sound you choose. The sound continues unabated throughout the duration, or you can make discrete sounds at different times before the spell ends.
So the way it would play out is the bard would use the Instrument as the material component for the cantrip. So they would keep playing for the duration on the cantrip that can play different parts of the orchestra.
Mind you, the only sound would be the real instrument and one illusion sound at a time, but by switching the sound quick enough, they might register in the mind as overlapping. DM discretion.
As the bard levels up, they can switch to Major Image