I usually use ground planes extensively top & bottom, but on a mixed analogue/digital board I need to have a Butterworth low pass filter - should I avoid any ground plane around this so as not to influence too much the L's & C's?
Make the separate ground for the digital section and analog, connect them together under your CODEC. Also make sure to separate power as well, and use atleast simple LC filter to power your analog section
1.Try to isolate the Analog and digital area, when you are going to pour the copper.
2. Also don't route any trace across the pour area. If you desperately wanted then, provide 3*Width of the line gap between the pour and your traces.
I hope this suggestion would improve your quality of the board without affecting the performance.