Posted: May 13th, 2011, 9:04 pmAs long as it's not a pay file, most artists are cool with it. If you're really concerned, ask first.
You should ask first. Legally the author owns the copyright and therefore the right to control derivative works. I think for most people that are a part of the community, it's no problem, as long as you're doing it for contribution to the community.
You might be able to do it without permission, but you're in a legal gray area.
Disclaimer - I'm not a lawyer, this is just my knowledge of copyright based on what I've learned over the years.