a) if something is available for download, it doesn't mean it's available for people to use at will. It doesn't mean it's copyrite free unless it says so explicitly. There is such a thing as (c) free music, you can buy CD's of the stuff.
it doesn't matter if your selling it - redistribution is a breach of (C) unless you have permision in advance.
c) just because something is available for download does not mean the author "doesn't care any more" - everything on the web retains copywrite unless the owner explicitly relinquishes it.
d) The author isn't asking you to advertise their work, and you can't do that with out permision in advance no matter what your good intentions may be. It's not yours to decide, it's the authors.
e) There is a slight grey area which you MAY get away with it under, which is "fair use". In some parts of the world (and only some), there is the expectation that the end user can use the artists work for some other things as long as they remain within a nebulous idea of "fair use". In the UK for example, this argument has been used to allow people to make backup copies of CDs they purchase. In some cases, if you sample a very small amount of a larger work, and then use that in certain ways, it can sometimes (if you're lucky and have a good lawer) be permissible.
Other than the very limited and tricky bit under e) you can't use someone elses work without prior permision. That's just the way it is, and you can't just decided to re-write the law because you think something is a good idea and you have your fingers crossed....
Stick to writing your own music for inclusion unless you're prepared to jump through the hoops to legitimately use other peoples work. Since you tell us you're good at it, this shouldn't be a problem anyhow.
