it's an often-made mistake where random generation / randomness is confused with probability distribution. Having said that, I don't know (as is in really don't know) if RF noise is unbiased; doesn't sound like it?
If you are talking about DC bias (as in, long term average of raw readings), then "unconnected audio input" is pretty likely to have it - it's easy to introduce via component tolerances, and there is no real reason to keep it exactly zero for audio purposes. But it's also pretty trivial to fix in software.
If you are talking bias in more general sense, then audio input noise is non-uniform in the frequency space, for example there is low-pass filter which filters out high input frequency, and it will affect noise values too. Good whitening algorithm is essential.
The good news however is there are many noise sources which are actually caused by quantum effects in electronic parts, and therefore completely unpredictable. Even if NSA recorded all RF noise, they still could not predict what the ADC will capture. (But they might be able to capture digital bits as they travel over the bus...)
