Hi Jack,
Let's separate this issue from OMPA-L137 EVM .Can you please post this question to Audio converter forum hope they will answer to your specific question about the AIC3106 codec and make sure all you’re settings are correct and what is the test setup to measure the noise level …etc.
http://e2e.ti.com/support/data_converters/audio_converters/default.aspx
Regards
Antony