Parkinson's patients experience a sensory processing glitch in the brain that leaves them unable to detect that they're speaking quietly. Unaware of the problem, they are likely to feel frustrated at being asked to constantly repeat themselves. They may even complain that their spouse needs a hearing aid.

Parkinson's disease also often causes a loss of facial expressiveness. That change, combined with a soft, monotone voice, can lead family and friends to think the patient is "just depressed, apathetic, bored, disinterested -- whereas the person inside feels quite alive," says Cynthia Fox, a speech-language pathologist and researcher at the National Center for Voice and Speech in Denver.

The upshot is that others stop engaging patients in conversation. They may come to feel ignored, and such patients often give up, says Fox. They drop job responsibilities that require a lot of phone talk, and they avoid dining out because restaurant noise further drowns out their voices. "It's a real blow to self-confidence," says Fox.