Voice AI Speech Rate Parameters Found Ineffective; Style Must Be Set During Training

Developers building voice models for roles like narrator, call center, and MC discovered that the synthesis API's speech rate parameter was accepted but had no actual effect on output. Testing showed speech rates remained virtually unchanged regardless of the parameter value passed, due to a server-side bug where the parameter was never forwarded to the synthesizer. The finding revealed that speech characteristics such as rate, intonation, and emotional style are largely determined by the training corpus rather than adjustable at synthesis time. As a result, the team redesigned their system to define 14 distinct 'speech profiles,' each trained on role-specific script sets tailored to use cases like sales, counseling, and guidance. The case highlights that documented API parameters must be empirically tested, as acceptance by an API does not guarantee the parameter is actually applied.