SigmaDSP projects have a software sample rate (set atop the SigmaStudio schematic) and a hardware sample rate (set by the HW Configuration registers). It's important to match these up -- otherwise, you'll find filter frequencies in error and perhaps worse problems. The first reference for resolving these issues is: FAQ: How do I change the sample rate of my SigmaStudio system? In addition, hardware on custom boards can introduce additional errors, even overclocking the DSP -- especially easy to do with the ADAU1701 (which seems quite happy overclocked by a factor of two, though of course not recommended). Avoid these problems by verifying that the various register settings, pin selects, and crystal frequency meet with data sheet norms.
Here's an easy test that spots most sample-rate issues: Place an oscillator in your project and output it to a DAC. Or output a pulse generator through a GPIO pin (while setting up the pin's HW config for output):
Scope the resulting output and see if it's the expected frequency. Often when using an evaluation board with a headphone jack, I'll just listen -- in my high school the "bell" was a middle-A (actually A -flat) tone, so to this day I can pick out 440 Hz a mile away. If your frequency is way off, find the cause before continuing to develop your project.