The Apple TV 4K uses a Dolby MAT encoder. So it takes the original Dolby Digital Plus audio from Netflix and plays it as Multichannel PCM. The Sony TV keeps the original audio as it is.

Thanks for the explanation GuitarSuperstar.  If I understand this correctly the DD+ is compressed and the PCM is not.  Why does the DD+ sound better?  It seems more crisp and dynamic to my ears.  

@Corry P wrote this on another thread:

“Differing content can often be mastered to different volume levels, and when a change in encoding is involved, this is even more likely as the sound track will be adjusted to best fit the parameters and abilities of the codec. Technically, however, LPCM isn’t a codec (it’s uncompressed) and is in fact how all digital equipment handles sound internally - at some point, regardless of any formats/codecs/channels being used, everything is converted to LPCM before being passed to the digital amplifiers inside all Sonos speakers/Amps. Included in the decoding process of Dolby Digital and Atmos to LPCM is a volume boost/range compression. As LPCM isn’t processed (decompressed) like a codec, there isn’t the same opportunity for this to happen. When LPCM is played, you get the closest to the original track possible - the data on the source is relayed directly to the amplifier after mixing.

Therefore, varying volume levels are normal...”