A text-to-speech (TTS) system implemented in an automotive vehicle is dynamically tuned to increase intelligibility over a wide variety of vehicle operating states and environmental conditions by tuning characteristics of the synthesized voice in response to measured operating states. To decrease distractions to an operator of the vehicle, an embodiment of the invention prevents updates to the synthesized voice character from taking effect while a message phrase is being played. Instead, voice characteristics are updated only during natural phrase breaks. In another embodiment of the invention, a damping filter is applied to calculated changes in voice characteristics to prevent excessively rapid changes from being applied, reducing the likelihood of distracting the vehicle operator. In another embodiment of the invention, both phrase-break detectors and damping filters are employed.