Text-to-speech as base64 audio. Default: MeloTTS WAV, up to 2,000 chars, in English, Spanish, French, Chinese, Japanese or Korean, $0.002. quality "high": Deepgram Aura-2 natural English voices as MP3, up to 500 chars, $0.02. Price: $0.002 in USDC per successful call, paid with x402 (Base, Solana or Arc); failed calls are not charged.