Thanks for your reply. We tried a few things after this post. Fs = 96kHz.
1) Tried inputting 20kHz into codec and looking at output on scope => 20kHz
Tried inputting 42kHz into codec and looking at output on scope => 42kHz
Incrementally increased frequency until rolloff => 44kHz (~1/2 96kHz)
2) On page 10 of datasheet, it gives frequency range of DAC output => 0.45*Fs. This says that if Fs=48kHz, rolloff will be about 21kHz. If Fs=96kHz, rolloff will be about 44kHz, which is what we saw.
So, it appears that the codec will have a max frequency output of 0.45*Fs (unless we're missing something).