According to Ashkenazim, music is prohibited during The Three Weeks. What's the source of the prohibition, and can one listen to Acapella music?
Interestingly, we don't find any early sources explicitly prohibiting listening to music. The Magen Avraham (551:10) states that one may not dance from Shiva Asar B'Tammuz but does not mention a prohibition against listening to musical instruments.
However, regarding the halachos of the day after Tisha B'Av, the Magen Avraham (558:1) mentions that music is prohibited until midday on the tenth of Av.
We know that the period after Tisha B'Av is more lenient than during the nine days, so if music is prohibited after Tisha B'Av until midday on the tenth, it must also be forbidden from Rosh Chodesh.
Another early source indicating that music is prohibited is the Pri Megadim (A.A. 551:10), which says that one may perform music for non-Jews for business purposes. We can infer from this that if it's not someone's occupation, they may not listen to music during this time.
R' Aharon Epstein in Shu't Kappei Aharon (52) explains why no early mentions of this prohibition exist. Since the Shulchan Aruch (560:3) rules that music is forbidden throughout the year to remember the destruction of the Beis Hamikdash, there is no need to state that it is prohibited during The Three Weeks because it is always forbidden.
Many poskim rule that even Acapella music is included in the prohibition. Shevet Halevi (8:127) and R' Elyashiv (Gittin 7a) mention that once the music is recorded, the music player turns into an instrument that plays the song, making it no different than a traditional instrument.
Another reason mentioned by R' Shmuel Kamenetsky (K.H. 4:4) and R' Belsky (Piskei Halacha 32) is that the prohibition is not against the use of musical instruments but against a musical experience that causes joy, and Acapella is included in that.