Between Passover and Shavuot is a time when it is customary for Jews to refrain from listening to instrumental music. Many Jews return to listening to music around Lag B’Omer.
Chabad.org explains these customs on their web site. Aish.com also gives a thorough explanation of Sefirat HaOmer restrictions. Following is an a cappella song that is listened to by some religious Jews during sefira. The beauty of real a cappella music as an expression of mourning is that it is notably incomplete. Whether we mourn the death of a loved one or of the Holy Temple, we feel a sense of incompleteness. Someone or something integral to our lives is gone. We feel that something important is missing from our lives. Music in which the orchestra has been silenced gives artistic expression to this feeling and level of consciousness.