Rooting your GNEX and installing custom rom and using volume patch called NexusLouder might be the best option.
XDA has pre-packed rooting programs and detail instruction. All you have to do is download it, connect your phone to computer and run it.
I took that route and I'm really happy with overall volume level. I don't have precise measurement but it became as loud as my old Mytouch 4G.
-NexusLouder
[MOD] [GNEX] BEATS+XLOUDER! 4.1.1 [21/9/12] JB/AOKP Louder Audio Improvement Patch - xda-developers