I just got this dvar halacha sent to my mailbox today:
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Halacha: Can a Chazan use a Sweet Goyish Medley during Davening, for ex. Kedusha or Hallel, to Inspire the Congregation?
This is debated amongst the Poskim. Some maintain it is permissible (Yechav’e Da’as 2, 5), while others hold it is forbidden and even an abomination to use Goyish tunes for our Tefiilos (Tzitz Eliezer 13, 12; Vayashev Ha’Yam 1). All agree, however, that melodies which the Goyim play or sing in their houses of worship are forbidden to be used in our Shuls (see also L’horos Nosson 10, 14)
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One thing people here quote is the idea of “nitzotzos”, of holy sparks being reclaimed. Kabbalah also has the concept of “klipos”, or tumah that can never be kashered. No matter how many chumros you slaughter a pig with, it’s still treif. There is also some music that can never be kashered. As for the exact gedarim, it can’t be given. Simon and Garfunkel composed many heartfelt tunes from their Jewish neshamos. Does that make the tunes kosher? Is the music of Felix Mendelsohn automatically kosher even if written for a church? Banning much of today’s Jewish music is counterproductive,though, and as long as it’s not a veidas znus (like the Jewish concerts at treif nightclubs), it’s better to let it go.