Igros Moshe (Part 4, Siman 84) prohibits the use of a loudspeaker on Shabbat, arguing that with every word a person speaks, a new formation is created, thereby violating the prohibitions of Makeh B’Patish (the final hammer blow/completion of a vessel) and Boneh (building).

In truth, the Chelkas Yaakov (Part 1, Siman 71 onwards), the Minchas Yitzchak (Part 3, Siman 38), and the Tzitz Eliezer (Part 3, Siman 16) wrote at length regarding loudspeakers, walkie-talkies, and similar devices. They ruled that even if the devices were connected to electricity prior to Shabbat and no light bulb is ignited during use, they are still prohibited due to “Uvsha Milta” (public noise that creates a disparaging atmosphere) and the Zilzul (degradation) of the Holy Shabbat.

Accordingly, the Shevet HaLevi (Part 1, Siman 66) and the author of Teshuvos VeHanhagos (Part 1, Siman 216) prohibited even a mechanical loudspeaker on the grounds of degrading the Shabbat.

I recall traveling to Los Angeles alongside our teacher, the Rishon LeZion, HaGaon Rav Shlomo Amar Shlita, to strengthen the Jewish community. In a particularly large synagogue, they showed him a mechanical loudspeaker that operated entirely without electricity, and he too prohibited it due to the degradation of Shabbat. Therefore, a loudspeaker is not comparable to a hearing aid, which those same mentioned authorities permitted from the outset (L’chatchila).