There appears to be some history which hasn't been made public. Whatever the background, without takeing sides it does appear that Sing Out is at least now making some effort to improve accessibility, whereas I can find no mention of this on the National Federation of the Blind's website (of which NABM is part), let alone an explanation of why they are taking legal action.

It seems to me that NABM are attacking the wrong target. They should be going for Zinio, who publish digital editions of many magazines, not just Sing Out. If there is a legal duty to make these accessible to the visually-impaired, then the NABM or NFB should be pressing Zinio to improve access to all their titles. If there is no such duty, then I don't understand why NABM think they have a case against Sing Out.

The easiest way for Sing Out to provide equality of access would be to stop publishing the digital edition. The most likely result of this action is not improved access for visually impaired but poorer access for everyone. At worst, there won't be a magazine at all, in any format.