This may be an iterative of a common question, though I feel the specific scenario justifies asking. Is there an established guideline regarding the longevity of copyright protection status for an unreleased piece of music? Assuming a musician has written a musical passage which went unrecorded and unpublished, would its copyright as an unreleased work still stand forty, fifty years on, assuming the author has not taken further steps to copyright its contents during that time? Would a third-party who produces a derivative of that work for commercial or personal gain five decades after its authorship be liable for damages to the author? Could damages be claimed in a personal use case? (ie: re-recording the work and publishing it electronically at no cost?)
Such a work might well still be protected by copyright after 50 years. The time that copyright protection will remain in force depends on the country where the work was created (normally where the author was living). In some countries it will also depend on when the creation occurred. In many countries such protection lasts for 70 years after the death of the author. Under US law recent works use the "life of the author+70 years" rule. Works created before 1978 may be protected for 95 or 120 years after creation, depending on the circumstances. See This well-known chart for details.
If the work is protected by copyright, preparing a new derivative work, or performing or distributing the original or a derivative work are all copyright infringement, and the owner could sue. This is true whether a fee is charged or the work is performed or distributed for free. Exploiting the work for commercial profit might increase the amount of damages available, however.
If the author has not published or exploited the work for 50 years or more, the author might not choose to sue, but that is entirely the author's decision. The author would have the right to sue, and given the facts as described in the question, to collect some damages. There is not enough information in the question to guess how large the damages could be.
In the US 17 USC 504 provides for statutory damages, which the court has significant discretion on the amount of, with no proof of economic loss. Specifically this law says:
(c) Statutory Damages.—
(1) Except as provided by clause (2) of this subsection, the copyright owner may elect, at any time before final judgment is rendered, to recover, instead of actual damages and profits, an award of statutory damages for all infringements involved in the action, with respect to any one work, for which any one infringer is liable individually, or for which any two or more infringers are liable jointly and severally, in a sum of not less than $750 or more than $30,000 as the court considers just. For the purposes of this subsection, all the parts of a compilation or derivative work constitute one work.
(2) In a case where the copyright owner sustains the burden of proving, and the court finds, that infringement was committed willfully, the court in its discretion may increase the award of statutory damages to a sum of not more than $150,000. In a case where the infringer sustains the burden of proving, and the court finds, that such infringer was not aware and had no reason to believe that his or her acts constituted an infringement of copyright, the court in its discretion may reduce the award of statutory damages to a sum of not less than $200. The court shall remit statutory damages in any case where an infringer believed and had reasonable grounds for believing that his or her use of the copyrighted work was a fair use under section 107, if the infringer was: (i) an employee or agent of a nonprofit educational institution, library, or archives acting within the scope of his or her employment who, or such institution, library, or archives itself, which infringed by reproducing the work in copies or phonorecords; or (ii) a public broadcasting entity which or a person who, as a regular part of the nonprofit activities of a public broadcasting entity (as defined in section 118(f )) infringed by performing a published nondramatic literary work or by reproducing a transmission program embodying a performance of such a work.
In short, the mere lapse of 50 years does not make the unpublished musical work free for use without permission.