Hacham Ovadia On Singing Songs of Non-Jews
Since this has become Inyana DeYoma, I will translate from http://www.halachayomit.co.il/displayRead.asp?readID=57
R' Yehuda HeHasid, in Sefer Hasidim, writes, "One who has a nice voice should be careful not to sing songs of non-Jews because it is a sin, and a nice voice was only given to him to praise the Creator, blessed be He, and not for sin."
It appears that his intent here is referring to one who sings non-Jewish songs with their lyrics, which are love-songs, similar to what the responsa of the Ri"f (Rabbeinu Yitzhak Alfasi, the rabbi of Rabbeinu Yosef Ibn Migash, who is the rabbi of the Ramba"m) writes, using the following language: "A Sheli'ah Tzibur who sings with his mouth the songs of the Ishmaelites and utters from his mouth vulgar words, we remove him from his post (of Sheli'ah Tzibur), and upon him, and anyone like him, it is said: 'she hath uttered her voice against Me; therefore have I hated her.' And this is brought down as Halacha in the Rema (Orah Hayim 53). However, to say holy songs and hymns in a tune that was composed for words of love songs, it appears that in such a case, there is no prohibition whatsoever.
And even though there are a number of Poskim (including HaGa'on Ribbi Mas'oud Rokah in the book Ma'asei Roke'ah), who hold that even when the song (i.e. lyrics) by itself is holy, for example, Kaddish or Kedusha or words of praise and thanksgiving to Hashem, still the non-Jewish tune ruins it, nevertheless, the opinion of many Ge'onei Yisra'el is to be lenient in this. And similarly, it is known from many Ge'onim who composed songs and praises based on the melody of the tunes of love songs, and among them are the song of the Bakashot, which were composed by the Ge'onim - Ribbi Shelomo Laniado, Ribbi Avraham Entebbe, Ribbi Mordechai Levaton, and many others.
And therefore, the main ruling according to Halacha is that one may be lenient in this to combine holy words with tunes of the songs of non-Jews, but nevertheless, it is preferred for prayer leaders to combine them with holy songs that are known to the congregation, even if these songs originated with non-Jewish tunes, since in the many years that have passed, the words of the non-Jews have already been forgotten, and holy songs have remained in their place. And, they have already gone out from an unclean domain to a holy domain - and such is the Minhag.