Lots of opinions, (OK, yeah, it's iterations of the same opinion. Shut up), on various aspects of giving away music or working for free.
Work Not Play. Charity supporting musicians and helping them get fairly paid
18 Ways Musicians Can Make Money. Guess what this is about? You've probably considered or are using many of these, but they're worth thinking about if not.
9 Times It's OK to Ask Musicians to Play for Free.Examples of some of the reasons you might be given to work for free. Including "It's for God" and "You can put out a donations jar"
What I Learned From Giving Away My Music For Free This Weekend, from One Working Musician. If you let people download your music for free, are you doing yourself out of income? Probably not. Anyway, this is one perspective.
When Asked To Play For Free These Musicians Had Sharp Comebacks. They're all witty and you should be prepared to steal them for your own uses. Just like how I stole the image here - that's how the internet works.
Watch What Happens When You Ask Non-Creative Professionals to Work for Free. It doesn't go well, because durh.
Why "Don't Play For Free" is Not Enough. Opinion piece about working for free.
It's Not My Hobby, It's My Profession from Stronger Unions.
The Working Musician Report. Facts and figures from The Musician's Union about working for free in the last year.