Here is R’ Moshe’s teshuvah. He is referring to Torah tapes. I don’t see why music tapes should be better.
http://www.hebrewbooks.org/pdfpager.aspx?req=920&st=&pgnum=70
This is in O.C. 4, 40-19.
The relevant, final sentence:
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It is clearly gezel, not (just?) masig g’vul, according to R’ Moshe.