Could a bard play an instrument and use a spell (minor illusion, prestidigitation, or major image) to produce the sounds of other instruments, effectively becoming a one-man band for the spell's duration for the purpose of preforming to a crowd?
If the answer is yes but not using these spells, then what spell would be needed? Anything from another casting class?
With minor illusion, a Bard can create an auditory illusion for 1 minute, and that illusion can be complex enough to carry out a conversation. There is nothing stopping you from mimicking any kind of sound.
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.
Of course, since you can copy any sound with a cantrip, it's fair to say the third level spell equivalent, major image, is also capable of doing this. However, note that the visual and auditory components of major image cannot be divorced. You create a visual illusion first, which can produce sounds on its own:
You create the image of an object, a creature, or some other visible phenomenon that is no larger than a 20-foot cube. The image appears at a spot that you can see within range and lasts for the duration. It seems completely real, including sounds, smells, and temperature appropriate to the thing depicted.
Since the illusion has to be visible, you can't make an invisible instrument playing the lute, so for purely auditory illusions, major image is a bit limited.
Off the top of my head, I imagine it's not too much of a stretch to suggest that a bard could use Mage Hand to play an instrument whilst they are also playing. If an Arcane Trickster rogue can use it to pick locks and delicately remove coin pouches... why not?
Also, while it's a bit of a stretch, Unseen Servant could also be used to add additional accompaniment. It can only perform simple tasks, but I'm sure it could manage to bang a drum or perhaps even play a simple repeated melody on a piano.
Neither of these require concentration either, so they could be used in addition to whatever your bard is doing with minor illusion or major image.
