Pre-recorded WAV files for branded greetings + transition speech. Upload PCM-WAV mono ≤30s; the runtime plays these instead of TTS on nodes configured with greeting_type="audio".
PCM-WAV mono, 16-bit, ≤30s. Convert MP3/M4A first with ffmpeg -i in.mp3 -ac 1 -ar 16000 -sample_fmt s16 out.wav or Audacity.