to flesh out a bit what Alistair was pointing out:
Only your lawyer (which I am not) can advise you about whether your use of copyrighted material is fair use or you are infringing. A few rules of thumb, though:
1) different countries have different rules. A piece may easily be in the public domain in one country and be still protected in another.
2) In USA, music published in USA before 1923 is in the public domain.
3) In USA, music published after 1923, for the most part, will be in the public domain several years after the death of the composer (I think is something like 95 years, not sure).
4) I have a vague recollection that in most European countries music is protected until after 75 years past the composer's death.
5) Generally, performing music in your home, for yourself or a small group of friends not for profit is fair use, regardless of whether your neighbors can hear you.
6) but if you go to a bar to play a gig and play the original or an arrangement of something portected (say, lovely "Happy Birthday") you owe royalties to the owner of the copyright (that is, the composer or someone who bought the rights from the composer) and you should have gotten a license in advance.
7) posting a private amateur recording on the internet, I think, goes beyond fair use, so I think it is great the forum got a blanket license agreement. I assume it only covers composers that have joined the licensing entity, but that is a nicety. If you play my music, please send me the check directly.


playing protected music in your lessons, auditions or even student recitals generally is fair use. Once you no longer are a student, though, it gets a little grayer.
9) the reference to the price of the book you buy, I think, is to the first sale doctrine: If you buy the book, you do not infringe copyright by reading the book, selling it, or giving it as a gift.
10) and the question about multiple authors and arrangements, I think, simply turns to what each author has a right to. Translators and arrangers do not have right to the royalties for the underlying work, but only for the aspect they authored.
THIS IS NOT LEGAL ADVICE, but just a little description of my limited understanding of the subject.
Cheers,