halacha - Learning music theory during the Three Weeks?

Would learning about music be allowed, if not actually playing an instrument? Would I also need to not listen to instrumental audio?

Or is that still itself an activity that could be considered enjoyable and not recommended? (implied underlying question: may one engage in any voluntary activities that he enjoys? Where should one draw the line?)

Answer

There are sources permitting learning to play an instrument until Rosh Chodesh Av, some even say until Erev Tisha B'Av. Those who permitted playing for real would surely permitting learning about music.

For instance OU writes

Similarly, She’arim HaMitzuyonim B’halacha (122:2) writes that one who is learning to play an instrument may practice until Rosh Chodesh Av. Rav Moshe Feinstein, zt”l was more lenient and permitted practicing a musical instrument even after Rosh Chodesh Av, preferably in a secluded place (Moadei Yeshurun p. 128:7b and p. 151:19). This permit is only until Chatzos on Erev Tisha B’Av (Orchos Chaim 552:10).

and dinonline writes

As Rav Moshe Feinstein writes (Iggros Moshe, Orach Chaim Vol. 3, no. 87), it is permitted to study music, or to teach music, when one does so for reasons related to one’s livelihood, and not for pleasure (see also Seder Pesach Kehilchaso, Chap. 12, no. 16; Tzitz Eliezer Vol. 16, no. 19).

Regarding listening to instrumental audio for the purpose of learning, and not for enjoyment, many but not all permit, see e.g., here from R Eliezer Melamed (under Listening to Recorded Music).

Of course, consult your rabbi before implementing anything you learn here.