Is one allowed to listen to “a capella” singing (mimics music) during sefira or the 3 weeks?
It is permitted to listed to vocals, or to “a capella” music (that sounds like vocals) in these times.
Rav Moshe Feinstein (Iggros Moshe, Orach Chaim Vol. 1, no. 166) writes that there are little grounds for permitting instrument music throughout the year, and explains that even if one is lenient during the year, he should at least observe the halachah during the sefirah period.
Based on this approach, there is certainly room for leniency concerning listening to vocal singing, without musical accompaniment, for the general prohibition of music during the year applies to instrumental music alone.
Thus Rav Moshe concluds that “In days of the sefirah it is prohibited, even according to lenient opinions, to listen to instrumental music.”
Although there are also stringent opinions (see Shevet Ha-Levi 8:127; Tzitz Eliezer 15:33), the general custom is to be lenient.