Phone-quality TTS: 8 kHz mu-law on Sume and where MAI Flash fits

Yes, Sume's TTS schema lets you ask for phone-style audio. output_format takes a container of mp3, wav or raw, a sample rate that includes 8000 Hz, and for wav and raw an encoding that includes pcm_mulaw and pcm_alaw. So a prompt for a telephony system can be requested as 8 kHz mu-law directly. The default is different: mp3 at 44,100 Hz and 128 kbps. Test one prompt on your own phone path before you render a library.

What the schema allows

These values are from the OpenAPI description of POST /v1/tts-1.0/generate. Not every combination is guaranteed to be offered by every voice, so verify by rendering one file.

How to request it

Send the encoding explicitly, and use wav if you also want sentence slices, because slice audio_url values are produced only for wav or raw.

{
  "transcript": "Thanks for calling. Press one for sales.",
  "avatar_handle": "@your_voice",
  "language": "en",
  "output_format": {
    "container": "wav",
    "sample_rate": 8000,
    "encoding": "pcm_mulaw"
  }
}

Where the call-center pitch fits

Microsoft positions MAI-Voice-2.1-Flash for call centers, voice assistants and IVR systems, at about 45 ms of model inference and $15 per million characters (read 2026-10-04). That pitch is about speed when the text is created during the call. If your calls play fixed prompts, speed is a smaller factor than file format and loudness, and a job that renders once is enough, as in the fixed prompts walkthrough.

If the text is generated live, Sume's TTS is the wrong shape: it is an async job you poll or receive by webhook, not a stream (see Jobs and results).

Checks for phone audio

Narrowband audio hides detail, so numbers and names can blur. Listen on a real handset, spell out digit strings with spaces or hyphens in the transcript, and keep volume in range with generation_config.volume (0.5 to 2.0) rather than clipping. If you join prompts into one file later, timeline audio requires parts that share a channel layout, so keep one format throughout.