Q. Why do (Orthodox) synagogues not allow the use of organ music?
In Britain it was mixed choirs, in the United States mixed seating, in Hungary the removal of the bimah from the centre of the synagogue, and in Germany the introduction of the organ.
Instrumental music was in use in the Temple in Jerusalem on Shabbat, but this was a specific exemption to the general halachah.
The rule is that it is not permitted to play an instrument on Shabbat on the basis of the prohibition against completing an object on Shabbat (Eruv. 104a). Maimonides explains (Hil’chot Shabbat 23:4; see also Shulchan Aruch, Orach Chayyim 338:1) that using an instrument “completes” it.
A further consideration is that if a person who wants to play an instrument repairs it on Shabbat, this is also a transgression.
After the emancipation, orchestral music in the synagogue was used for state occasions held on weekdays, and instrumental music became customary at weddings.
But halachic authorities strongly resisted the use of the organ on Sabbaths and festivals, and in some cases a candidate was refused rabbinic ordination unless he undertook not to officiate at an “organ” synagogue.
In 1819 a rabbinic publication called Eileh Divrei HaB’rit ruled against playing the organ on Shabbat which was introduced by the Hamburg Reform temple, and stated that as well as a Jew not playing the organ him/herself, a non-Jew must not be instructed to play it.
Some even banned the organ on weekdays because instrumental music introduced a feeling of rejoicing that was inappropriate since the destruction of the Temple.
They added that worship with the organ had become chukkat hagoy, a gentile custom.