Is this true? If you crank the gain on the cheap mic amplifier that comes with basically any laptop or motherboard, you'll get a ton of noise. Yes, the high bits will be correlated between samples, but if you XOR all the bits in a sample - or multiple samples - together, that ought to be a pretty solid random bit, no? You just need to make your XOR-ing "window" span at least 1 bit's worth of entropy in the stream. It's not megabytes per second, but it's a lot better than having the user wiggle the mouse, surely?
Analog TV tuners were very cheap. Now it no longer makes sense to use an expensive analog capture board for a RNG.
There are much cheaper solutions.
For no more than $10, you can make a RNG using the ADC of either a microcontroller board with USB interface or of an audio USB dongle or of the microphone connector of the motherboard audio.
There is no need to tune to a dead channel. Instead of connecting a TV cable or antenna, it is enough to connect a coaxial resistive terminator on the TV input connector. Then all the channels are dead.
Unfortunately this is a dead technology, so I have abandoned it after I retired my last motherboard with PCI connectors.
You can do something similar with the audio microphone input, but the TV tuners had a much higher bandwidth and a much higher sensitivity. For good results with the microphone input you need to make an analog noise generator, instead of using just a resistor, as it was enough for a TV tuner.