Google Pixel phones feature the Qualcomm Aqstic audio codec. This codec was developed to produce richersound and high-dynamic range and high-resolution playback up demanded by sophisticated listeners, and everyone who actually wants a phone that can deliver a high-quality listening experience. This codec is also featured in the HTC 10 and Xiaomi Mi5 and the reviews just praise their audio quality and the Google Pixel won't be any different as far as its call quality and headphone listening quality goes.
The Qualcomm Aqstic audio codec also bears dual oscillators so your music files can be processed at native sampling rates without going through any conversion processes that can degrade the audio quality.
Added to that is thenew Qualcomm Fluence Pro technology. This tech was developed to help improve the call quality by employing a series of innovative noise cancellation technologies.
All these technologies help improve the audio quality for both calls or when consuming multimedia files and to promote excellent listening experience compared to previous technologies used in earlier devices.
Although the Google Pixel and Pixel XL lack stereo speakers, you can rest assured that you get excellent call quality and audio quality when using a high-quality headset or earbuds. I hope that Google will consider adding Stereo speakers to the second iteration of the Pixel devices so we can enjoy high-quality audio from the built-in speakers.