AVAudioConverter keeps only channel 1 when downmixing stereo and outputs
pure silence for layout-less multi-channel inputs (e.g. a 4-input USB
interface such as the UA Volt 476P), so the dictation model heard nothing
and the HUD never showed text. Pick the channel that carries the voice
(loudest on smoothed energy, switching only on a clear 6 dB margin) and
convert it as mono. Also reserve the dictation locale, which the Speech
framework warns will become required, and log the mic format.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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