Unfortunately this is a system that is abused in the USA a great deal. (Like most other things done in the name of capitalism greed) But I still want to know are you making good money with this busy originals band? Are these gigs paid or not?
As an unpaid intern, if you hope to transition into a paid role with this particular company, keep a careful eye on your sick time and look into the possibilities of catching up with the band as mentioned before.
That said, as an unpaid intern, your job is not to fulfill the duties of a paid employee. You are meant to be learning how things work in your desired field and exposed to a wide array of the day to day. Your employer could actually be stripped of the privilege to staff unpaid interns if it were found out that your duties were identical to a paid employee.
Internships are not meant to provide companies with free labor and companies have been sanctioned by the labor board for replacing paid labor with unpaid interns.
Sorry for the rant, labor and education reform are some pet causes of mine.