Where is the line drawn? What's protected by the movie authors and what's fair game?
Also-- is there a forum for this kind of question?
What is protected is not as easily determined as it might seem. But limiting discussion of 'protected' to copyright only (as has been pointed out commercial product can have other forms of actionable 'protection'). A performance is automatically copyright(ed). This covers any mechanical reproduction of that performance. An audio recording of the performance, a video recording of performance, used outside of fair use restrictions are fairly obvious violations. Though what is or is not 'fair use' is still an open can of worms and subject to significant legal debate. The question gets a little muddier when you are dealing with a 'digital' transcription that in its native or binary state bears little or no resemblance to original. A MIDI transcription (audio2MIDI) of a performance seems to fall on the side of copyright violation, while taking the same type of data, a digitized audio performance, opening it in a graphic editing app, printing and selling the result is a little less clear cut (concerning copyright infringement). Second type of copyright, not necessarily automatic, are the mechanical foundations for the performance. Words of the text; some form of notation substantiating some form of musical element (does not have to be melody), from which the performance can be reproduces; choreographic notation; etc. There are legal debates concerning whether a specific staging of a theatrical production can be protected with copyright. Essentially, while not limited to, anything that can be commercially published can be protected via copyright.
The changes that digital processes have brought to distribution have so far only added confusion and chaos to the legal arguments surrounding 'intellectual property'. But a simple rule of thumb is that if something that you think of as yours makes money, somebody, somewhere is going to be examining it to see if you owe them some form of compensation. In the scenario OP describes I do feel they are relatively safe copyright infringement action. In terms of things like 'brand dilution' perhaps not so much. But even here it largely depends on whether item comes to someones attention and how much revenue item in question might generate. But even in cases where revenue from offending item is quite small it is not uncommon for a challenging entity to try to force a 'cease & desist', if only to further protect what they perceive as their protectable rights.
Neither questions, nor responses, regarding copyright are necessarily simple or easy. While forums devoted to various legal issues exist, a site devoted to
copyright issues, I'd be even more leery of advice on those sites then ones dedicated to which mic to use or interface to buy. Unfortunately to address copyright issues you need to talk to a lawyer, someone bound to you by contract, that you can sue when their advice turns out to be shit.
To summarize: accepting OP's description @ face value there does not appear to be any 'mechanical' connection between inspiration & OP's product that is protected via copyright. While I'd have no problem ignoring any infringement possibility if I had composed, recorded the work, to get a slightly more dependable opinion one has to confer with a lawyer, with copyright experience.
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