"Have to be" or "are to be": difference in meaning?

Given that it is a necessity that both instruments sound true during the show, we could say that the sentence is trying to express the urgency of tuning the instruments. Therefore, it can be that "have to be" is most appropriate in this context, although both versions are grammatically acceptable. As Dan added, context is lacking to precisely pin down the correct one.

Looking at difference in meaning between have to be and are to be:

• have to be expresses a prerequisite for the show. If the instruments are not tuned, the show can't start. It is therefore closely related to "the instruments must be tuned" (but not the same)

• are to be expresses a lower sense of necessity that the instruments are tuned, and rather indicates that doing so is general procedure. It has less emphasis on the "must", and instead it seems to convey that "this is always the case so the instruments will also be tuned this time". Like Grizzly suggested, it is expressing the expectation that the instruments are tuned.