start.mediaFormat.encoding: The encoding of the data in the upcoming payload. Value will always be audio/x-mulaw
start.mediaFormat.sampleRate: Sample Rate: The Sample Rate in Hertz of the upcoming audio data. Value is always 8000
Standard
Best Practices for Audio Recordings
The telephony standard is 8-bit PCM mono uLaw with a sampling rate of 8Khz. Since this telephony format is fixed, any audio file uploaded to Twilio will be transcoded to that telephony standard. That standard is bandwidth-limited to the 300Hz - 8Khz audio range and is designed for voice and provides acceptable voice-quality results. This standard isn't suitable for quality music reproduction but will provide minimally acceptable results.