Piper voice: en_GB-northern_english_male-medium
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Engine: piper-tts 1.4.2, GPL-3.0-or-later, The Home Assistant Authors.
Upstream: https://github.com/OHF-Voice/piper1-gpl
Engine license: PIPER_GPL_3_0.txt (separate from voice dataset terms).

The upstream voice model card identifies its dataset as OpenSLR SLR83:
Crowdsourced high-quality UK and Ireland English Dialect speech data set.
Dataset: https://www.openslr.org/83/
Dataset copyright: 2018, 2019 Google, Inc.
Dataset license: Creative Commons Attribution-ShareAlike 4.0 International.
License: https://creativecommons.org/licenses/by-sa/4.0/
Publication authors: Isin Demirsahin, Oddur Kjartansson, Alexander Gutkin,
and Clara Rivera, "Open-source Multi-speaker Corpora of the English Accents
in the British Isles" (LREC 2020).

PHNTM One uses this voice for on-device speech output.
The upstream model card records fine-tuning from U.S. English lessac medium:
https://huggingface.co/rhasspy/piper-voices/blob/main/en/en_GB/northern_english_male/medium/MODEL_CARD
These dataset terms and training provenance are separate from the engine license.