UPDATED ANSWER
I've reproduced this issue when userPrompt has two strings, e.g.) text area contains line breaks.
You might add join command to combine multiple lines into one string array.
userPrompt = app.whatdoyouwanttosayTextArea.Value;
userPrompt = string(userPrompt);
userPrompt = join(userPrompt); % combine multiple lines in to one string array
NET.addAssembly('System.Speech');
speak = System.Speech.Synthesis.SpeechSynthesizer;
speak.Volume = 100;
Speak(speak, userPrompt);
ORIGINAL ANSWER
Are you using another .NET or Java library which also uses Speak function?
How about the following?
speak.Speak(userPrompt);
