You're overthinking this making things (in your mind) more complex than they are.
Suppose we have a 10-bit ADC, then the will be roughly 60 dB (yes, theoretically 61.96 dB but I like round numbers, also: feel free to show me an ADC that works exactly according to theory, but I digress). So SNR = 60 dB
Let's say that this ADC has a full scale of 0 dBV, that's ...
An 8-bit audio signal has a theoretical maximum SNR of 48.16 dB
On the face of it, an 8-bit digitization process would increase "noise" by 3 dB (as per Brian Drummond's comment). However, when you take into account the natural "crest factor" for audio, the "noise" problem is worse: -
Image from here.
would an analogue audio (...
Can I expect alias on the signals used in the control task of 20 kHz?
Yes. The 500kHz signal could have have signals up to 200kHz in it. It would get aliased when re-sampled for feeding into 20kHz.
Should in this case a digital filter at the sampling rate of 500 kHz applied that has a maximal cutoff frequency of 10 kHz (Half of control task rate)?