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.