by chung on January 26th, 2011, 12:08 pm
It goes back to what you're allowed to do with material in the public domain. Something that it's in the public domain can be taken by someone other than the original creator and altered as that person sees fit. Cleaning up a recording would be a form of alteration or editing and that person is allowed (usually) to put a copyright on his/her work. It matters little if an end-user doesn't perceive a change. This is how companies such as Multilingual Books among others can put copyright on the FSI materials in its inventory, as long as it makes some kind of change to the original material (be it a cosmetic one such as a different font or larger typeface for people with poor eyesight), or something more substantial (e.g. updating the content or rearranging the sequence of the sections), and can provide documentation or proof of its editing work to defend its copyright, if challenged.
This is somewhat like recorded music (especially most classical music). The scores of someone like Beethoven are in the public domain (the idea is that a composer's manuscript/score is in the public domain starting 75 years of his/her death). However that doesn't exclude people from taking the score, recording the music from the score (or in some cases even altering it for whatever reason, as some conductors do), thus allowing these people to put a copyright on the new output. The fact that this kind of music is in the public-domain allows for anyone to perform it (e.g. orchestras don't sue each other for performing "Ode to Joy").