Pronouncing Hard Words: Phoneme Adaptation of a Pretrained Conversational Text to Speech Model

Voice agents built as cascaded automatic speech recognition (ASR)->large language model (LLM)-> text-to-speech (TTS) pipelines increasingly rely on conversational TTS: streaming, context conditioned models that adapt delivery to dialogue history. Such models are natural and expressive but offer little explicit control over how rare, trust-critical drug names are pronounced, because these entities are too infrequent, and their vocabulary too fast-growing, to be covered by any amount of pre-, mid-, or post-training. We retrofit phoneme controllability onto a pretrained conversational TTS model without any pre/mid-training, using an ARPAbet vocabulary exten-sion, continued phoneme-augmented supervised fine-tuning (SFT), and a training-free inference-time acoustic-biasing step. On a 7,500 clip clinical drug-pronunciation evaluation, our model raises strict Gemini-judged correctness from 54.6% (no phoneme augmentation) to 90.7%, outperforming every conversational and stateless TTS baseline on the automatic metric. Because no training stage sees a drug name, the entire evaluation is out-of-domain, so the gains reflect learned pronunciation rather than memorization. In a blinded, randomized human study, adult listeners judge 84% of our clips correct, versus 82% for the strongest baseline, with overlapping confidence intervals on a stratified subset. Across all 250 matched clips, Gemini agrees with the human majority on 84.8% of binary labels (k appa =0.57).