How normal would it be for parkinson's patients to mess up their speech?

Speech issues mixed: There is no question that parkinson patients can have speech issues. There can sometimes be mild slurring of speech (dysarthria). Frequently parkinson patients have voice changes, with the voice getting much softer and changing in tone. A speech therapist can be very helpful for these problems. As far as "messing up speech", if you mean by saying wrong or nonsensical words, that would be unusual.