Is anyone aware of any halakhic opinion regarding the playing of musical instruments, guitar for example, in shul on a weekday, not Shabbos. Does it have the same religious stigma and opposition as having an organ in shul did at the beginning of the reform movement? Is it still seen as a move towards the practices of other religions?
In describing the synagogues of Bavel in the Twelfth Century, R. Petachia of Ratisbon wrote as follows:
בחולו של מועד אומרים המזמורים בכלי שיר
On the half-holidays they recite the psalms to the accompaniment of musical instruments,
(Travels of Rabbi Petachia of Ratisbon p. 46-47)
It seems like the rabbinic authorities there allowed this, and no objection by R. Petachia is recorded in the travelogue.
See this article. Excerpt:
The classic Talmudic source on musical instruments is the Babylonian Talmud (rabbinic text finished in the year 500 and edited until approximately 650 C.E.), tractate Beitza 36b, in which the rabbis explain that the rabbinic prohibition is based on the concern that one might end up fixing the musical instrument if it became necessary.
In the nineteenth century, when Reform congregations started playing instruments during synagogue services, Orthodox rabbis issued strong directives against this practice. Rabbi David Zvi Hoffman (1843-1921), the rector of the famous Hildesheimer yeshivah, explains the evolution of this prohibition in his responsa Melamed Leho'il (first part, Orah? H?ayim, chapter 16). He writes that the Rabbinic Court in Hamburg published a series of letters by different halakhic authorities under the name of Eleh Divrei Habrit in 1819 in which they all prohibited playing musical instruments (particularly the organ) in the synagogue even if the instruments were played by a non Jew. This was a reaction against the Reform synagogues' custom of playing instruments during services.
He notes that the halakhic authorities were not clear about playing musical instruments on weekdays. Some of them did not address the issue and might have allowed it, while others permitted playing instruments during weekday services. Other authorities cited definitely prohibited this practice. Noting that Jews are not permitted to follow Gentile practices, in 1820, Rabbi Abraham Lowenstein, the head of Emden's rabbinic court, prohibited playing an organ in the synagogue on Shabbat, Holidays and weekdays because we are not permitted to follow the Gentiles ways.
Beginning in 1863, many rabbis relied upon this proscription, and in fact the Hildsheimer Yeshivah in Berlin ordained rabbis on the condition that they would not serve in synagogues with organs (in any service. Organs were singled out as both churches and Reform synagogues used this instrument). Rabbi Hoffman asserts that the musical instruments in the Temple were not like the organ. Moreover, he noted that the organ used in the Al Tnai synagogue in Prague was played before Shabbat started. He provides three explanations for prohibiting instruments at synagogue services: a) Not to follow the Gentile practice b) Not to follow the heretics' ways c) Not to forget our grieving for the Temple's destruction
My added commentary - Shlomo Carlebach often played his guitar during Havdallah in the shul. I've asked elsewhere about this.