In my Sefira posts I mention a number of Jewish acappella albums and link to some YouTube vidoes of acappella performances. Here are two more. The first is 8th Day, one of my favorite orthodox/chassid pop groups. The second is from Hassidic rapper Y-Love and beat-box artist Yuri Lane.
Y-Love & Yuri Lane Purim Beat Box Freestyle for Sefira
You can also listen to acappella group SIX13 recorded on the JM in the AM show, pickup lots of Jewish Sefira-friendly acappella albums at JewishJukebox.com, and check out these sites for more information on the Three Weeks:
1. Chabad.org "The Three Weeks and Tish Ba'Av"
2. OU.org "A Chronology of Destruction: The Three Weeks"
3. Aish.com "Tish B'av and the Three Weeks"
4. YU Torah Online "Rabbi Hartstein- Music, Shaving & Haircuts in The Three Weeks" (A recorded shuir, or lesson)
5. VBM Torah Studies "The Three Weeks and the Nine Days"
(Hat tip to The Jewish Music Review for the 8th Day video and the JM in the AM link)