I also noticed that when I was singing with a choir my voice was stronger than when I simply sang folk music. Choral singing has different demands. I need to keep myself working the voice, NOT FORCING IT, so that it sounds better in the folk songs as well. But sometimes I slack off because I don't have to be at rehearsal as regularly and there's no choir director to tell me what to do. Also sitting down with the guitar is different from standing facing the conductor. I need to remind myself to sit up straight and not hunch over watching my fingers. Apart from that there were some threads on singing and the voice back in early September. I can't off the top of my head remember their titles, and dinner has suddenly appeared, but if you search about that time you should find some useful information.