For years I have been told I have a quiet voice. My husband cannot remember it being any other way. I attributed it to speaking more softly when unconfident or being in new situations, and not having a very resonant "timbre" to my voice. I fully expected though that my "angry parent" voice could still carry to the back of a room.
In 2012 I became aware of having to make an effort to speak more loudly if I wanted to contribute to a group conversation, otherwise it would just carry on without people hearing me. I found myself not contributing as much to the group, preferring instead to wait till I could talk to people one-to-one.
After my trip to Australia at the end of the year, diagnosis, then return to Oregon in April, I went to my local support group and found that most of the members could not hear me. At subsequent meetings, I consciously worked on projecting my voice and speaking louder, and I eventually changed my seat to be closer to them.
My karaoke voice disappeared too. I mourned this for a time, then read up on speech therapy. It seems that singing is good exercise for the voice, so I have got into the habit of singing in the shower, around the house, in the car and even (very quietly) when shopping.
A singing group meets weekly at the local senior center, and two of my friends are members, but so far I have held off going.
Take a deep breath before speaking.
Saying letters loudly
Sit in front of a mirror, place your hands on your throat, take a deep breath and with all your strength shout out Aaaaaaaaaaaa using one continuous breath and continue with the Aaaaaaa until you're out of breath whilst bending forward and resting your head virtually on your knees.
Do about 6 letters a day once or twice a day until until you have got through the alphabet. This exercise will strengthen the voice box and surrounding muscles whilst educating the conductors in the brain how your voice box should be working to make sound.
Join a singing group.
lee Silverman Voice Technique (LSVT LOUD)
I haven't tried this yet, but hear that it helps.
Shake, Rattle and Roll: Parkinson's: Communications, Speech and Voice - Kate Kelsall's blog which includes speech problems and what she has done to improve her voice.
Voice Aerobics book by Mary Spremulli - 3-part voice exercise program (breathwork, powering up your voice, and cool down with good vibrations), includes CD (or DVD) of voice exercises. Also available from your local library.
Singing allows people with Parkinsonís disease to exercise their vocal cords - Phillis Richman's story of how signing up for a singing group helped her with voice therapy and much more.