At the end of the day, consult your local Orthodox rabbi rather than wall-posters. And perhaps consider the words of the Chafetz Chaim against pashkevils:
There you have it. According to the Chafetz Chaim, Rav Amnon Yitzchak is committing a great sin in printing up and promulgating this kol koreh. And he is machti es harabbim by causing such machlokes. I think it would be fitting to hold ceremonies smashing his CDs, just as he encouraged others to do.
my translation of the pashkevil.
But anyway, here is why frum Jews should not pay any heed to these kol koreis. (Also, please refer to my previous discussion of Rabbi Amnon Yitzchak's attempt).
1) We saw a video of his attempt to persuade Rav Shteinman to sign on to his ban.
Here is a video of this meeting.
(And see this video of an askan manipulating Rav Elyashiv.) In the video above, Rav Shteinman does not agree. But it is clear that Rav Shteinman does not know what a CD is, who Shwekey is, etc., etc. And Rav Amnon Yitzchak is giving false or misleading information. Meanwhile, these frum Jewish singers are not given the opportunity to present their side. After having seen this video, and Rav Amnon's Yitzchak's contemptible (IMHO, but that is a matter of opinion) actions, how can you believe this kol korei? Until I see similar unedited videos of all of his meetings -- whether subsequently with Rav Shteinman, or with Rav Elyashiv, or with Rav Kanievsky, there is no reason to grant any credence to the kol korei. A rule in computation is GIGO, Garbage In, Garbage Out. To cite Charles Babbage:
These Gedolim may well be great talmidei chachamim and so on, and will advise you of the correct action to take given an accurate description of the metzius. But if you feed them false information on the metziut, and don't allow the other side to present their version of the metziut, then what will come out will be garbage. Just as if I ask a shailah about the kashrus of a chicken but really I am asking about a parrot.On two occasions I have been asked,—"Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?" ... I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.—Charles Babbage, Passages from the Life of a Philosopher
2) Manipulate one gadol to sign it and, unfortunately, it is likely that other gedolim will sign on as well. When Rav Kanievsky accidentally signed on to something promoting the innocence of Rabbi Elior Chen, his explanation was: