Do you have the 2 or the 2XL, and are you talking only about headphone audio?
The reason I ask is that the headphone adapter, like the iPhone 7/8 one, contains a DAC and amp, so you'd expect the output to be the same from the Pixel 2 and the 2XL (as it's the same adapter). But I remember that PhoneArena's review showed the 2XL giving a much lower output than the 2. However, I don't know whether this is typical, anomalous, or not true at all. The reason I say the last is that according to
this XDA thread the Google headphone adapter adjusts its output voltage according to the impedence of the headphones plugged into it. That isn't daft, since low-impedence headphones don't need a large voltage to drive them. Now you'd think you'd have to be an idiot to use different headphones to compare two different devices, but I can't help notice that the voltages
measured by PhoneArena for the 2XL and the 2 respectively are very close to those measured by the XDA member for low-impedence and mid-impedence headphones, which does suggest that PhoneArena may indeed have idiots doing their reviews
.
The relevance of this is that if you are using low impedence headphones and are comparing to a device which doesn't adjust the output voltage this way they could sound quieter.
I have a Pixel 2 (non-XL) and don't have any such problem, but hopefully some 2XL owners will comment as well.
One feature of using USB-C for the audio output is that using a different adapter should change the output (I mainly use a HTC one, which with my phone is marginally louder than the Google one), though I've not yet got a clear picture of how much variation you get when using the same adapter with different phone models (I've only the one I can test it with myself).
A final thought: you could experiment with using a different music player. Neutron certainly has different options for driving external USB DACs, which is what these adapters actually are, so it might be worth playing with that to see whether it sounds any different.
