It has become trendy to accompany mid-week Hallel prayers, such as on Rosh Chodesh or Chol Hamoed, with musical instruments and dancing.

Rav Yehoshua Van Dyke, rav of Ramat Magshimim and rosh yeshiva in Itamar, has issued a psak that it is prohibited and inappropriate to play guitar, or other music, during Hallel.

According to Rav Van Dyke, this goes against the tradition, generations strong, of not having musical accompaniment in shul. Some rabbonim, such as Rav Yaakov Ariel, allowed music and dancing, but only after davening concluded.

Rav Van Dyke clarifies that he is not concerned that bringing music in makes it appear similar to either the Reform or even Christians. Rather, it is based on the psak of the Chasam Sofer that music is prohibited in shul out of a sense of mourning for the destruction of the Temple.

In addition, piskei halacha from Rav Chaim Falaji and from Rav Soloveitchik prohibited music in shul as a breach in the seriousness expected and appropriate for prayer.

Rav Eyal Yakobovitz, Rosh Yeshiva in Tzfat, disagrees with Rav Van Dyke and says that singing and dancing with music in shul is not out of forgetting the destruction but out of pining for the rebuilding of the mikdash. Singing and dancing and music accompanying hallel arouses fervor and inspiration. Rav Yakobovitz points to a number of communities throughout history that did so.
source: Kipa