DIY wifi radio radio NOISE!
This may or may not fix the problem that you asked about, but instead of hacking the power to a bus-powered hub, I think you should use a self-powered hub that has its own correctly-designed AC adapter.  If you have to power the system on 12V, you can then look at what the AC adapter provides and derive that from your 12V supply.

Also, I'd be tempted to use higher-valued resistors for the passive stereo->mono mixer.  More like 5k-10k for the mixing resistors and skip the ground resistor entirely.  The amp probably has one already.  I'd be nervous about 470Ohms overstressing the sound card.  Much better than the short that I've seen from countless non-techies, but I'm not sure it's enough.