Some people are not accustomed to talking much. If you have been sitting at a desk, quietly banging on your keyboard until your fingers are falling off, and then you suddenly switch to talking all day, letting your fingers recover, you may experience Voice Strain. When it first hits, you might call it hoarseness. I'm sure some doctor will send e-mail to me and give me a nice technical name along with a $20,000 treatment regimen that won't be covered by your insurance. In any event, it can be serious.
Similarly, some of us never learned to speak clearly.
There are people who are experts in speaking. You may have avoided them in school as they were the dreaded "speech teachers." They have some good lessons for all of us. If you live in Hollywood or New York City, you can probably open up the phone book and find listings for Voice Coaches -- likely unemployed actresses and actors who figured out they could make money telling you how to talk. You'll pay dearly but you'll get one-on-one instruction.
Your community college has courses on speaking, and groups such as Toastmasters can help with speaking. But they tend to concentrate on the content, not the mechanics of speech.
There is a videotape, "A Voice of Your Own" from Patsy Rodenburg, Director of Voice at England's Royal National Theater (naughty Dragon -- should be "Theatre"), published by Applause Books ISBN 1-55783-284-6. US Price $40 -- probably higher in the UK. She demonstrates healthy voice habits, good breathing habits, points out some bad habits that cause problems, and gives exercises to overcome problems.
"The Art of Voice Acting" is a book for voice-over actors by James R. Alburger which includes a chapter on "Taking Care of Your Voice". From Focal Press ISBN 0-240-80340-x, about $20.
Regretably, I don't practice what I preach above... some day I will read the book, watch the video, and develop a voice like James Earl Jones. But until then I'm stuck with my own voice.