As far as I know nuclear decay is the only easy genuine random source available
Completely wrong. Others are thermal noise (in analogue electronics), and turbulence (in airflow). Your audio input jack and hardware can be a very effective random source. For best effect, connect a thermal noise source instead of a microphone: it's trivial to build one from a few discrete electronic components, and power it off a USB port.
But even a microphone listening to background noise will do. Even if the spooks have a hi-fi uncompressed bug in your office, it won't be recording exactly the same audio stream. The least significant bit per sample will be random, which is quite a reasonable source of entropy to blend into an entropy pool. (If you stick your random noise microphone to your PC's fan grille, it'll be more than one random bit per sample).
Finally, for an entropy pool you don't need random in the sense of passing all statistical tests for a random source. It just has to be non-reproducible and not remembered by anything. So the "signal" bits of the background noise in your office also qualify to a greater or lesser extent.