"When I asked my grandparents (who,whom) we were going to hear at the concert, I was excited to hear them respond with Midora's name."

Your first step in addressing this should be to realize that there are two clauses in the sentence:

Subordinate clause: "When I asked my grandparents (who,whom) we were going to hear at the concert, "

Main clause: "I was excited to hear them respond with Midora's name."

Your question pertains only to the subordinate clause. Thus remove the subordinating conjunction and look at the clause as though it were a complete sentence:

I asked my grandparents (who,whom) we were going to hear at the concert

This is an example of reported speech. Thus you remove the elements that are not part of the reported sentence to isolate the reported sentence, leaving:

(who,whom) we were going to hear at the concert

First note that "were going to hear" is a rather ugly colloquialism for "would hear", a future form of "to hear." "Who/m" would be the direct object of "hear", and thus needs to be in the accusative case, "whom."

NB: I corrected the apostrophe error in the original question as well.

The simple answer is whom. The way to figure out which pronoun to use is to rephrase the question.  Find the subject of the clause who/whom belongs with (we)and rewrite it as a separate sentence using a personal pronoun as a substitute. We were going to hear (her,she) at the concert; in this case, you are using the object form of the word her as the word is the object in the sentence.  Then go back and use the object form in the sentence you are asking about which is whom.  So, the long explanation is, we were going to hear whom.  Now put the whole sentence together.  When I asked my grandparents whom we were going to hear at the concert, I was excited to hear them respond with Midora's  name.  I find that if you use the personal pronoun to figure out if it is a subject or object form, it is easier to figure out the who/whom of subordinate clauses.  I do hope my explanation is clear enough to understand.