I am reminded about what R’ Belsky zt”l wrote regarding A Capella music:
Lately, it has become a trend to take every possible pleasure that one can think of and figure out ways to make them permissible at all times. Whether it is the imitation of non-kosher foods, making all chometzdike delicacies kosher l’Pesach, or other similar things, we find this attitude now more than ever. People cannot live for one minute with compromising on pleasures that they are used to or wish to experience. Often, the heteirim to permit such activities are, at best, based on very weak reasoning.
One such example is the desire to listen to music during Sefirah and The Three Weeks. It has become a trend to produce “Sefirah tapes,” referred to musically as “a cappella“. The wide acceptance of such tapes has not been with rabbinic approval. Indeed, many of the gedolei rabbonim have ruled that one should not listen to this type of music during Sefirah and The Three Weeks. Unfortunately, because the music albums are being sold in the stores, people think that they must be glatt kosher. If they aren’t acceptable, people say, why would a Jewish store sell them?
His words regarding this likely apply to massive siyumim/events as well. The 9 Days are a time for mourning – not finding ways around the rules.