Hey andy, long time no see...
This is exactly the problem I have with my setup. Although mine uses different ADC/DAC's I'm getting noise.
At first glance that input looks fine to me. Others have suggested that 'digital noise' could be the problem, and undoubtably there WILL be digital noise...but for it to be loud enough to drown out the actual signal..when the power supply is decoupled as in your schem? Not likely. At most there'll be an annoying hiss.
As for debugging, its a bit difficult to do it with a complete system. That is
analogue in -> buffer -> amp -> ADC -> digital stuff - > DAC -> analogue out.
So, if you assume that the DAC is getting the right information, in the right format, and the ADC is working fine. Try grounding the ADC input. Effectively that should output 0's for the sample, and the DAC should output 0v. Therefore you would only be left with noise from the analogue parts.
Also, I don't want to get into another issue here, but if the output of your DAC is say 0-5V...what are you plugging this into to 'hear' the output? One silly thing I did with my setup was forget to attenuate the signal before putting it into an amp...so I heard noise..but that was just low level noise, I just so happened to be amplifying it 20x so it clipped the amp. No audio was detected because of the same reason. So, I shall dig up the data sheet you provided for the DAC board, and have a butchers at the output circuit.
If you could remind me of the part numbers (or even link the datasheets again) in case I can't find them, I would be grateful.
Scott.
Ps. you got a provate message.