To me, this is the only one that could impact sound. Allowing electrical noise through from one point on the network to the DAC would allow for theoretical degradation of the output from the DAC. This is supposed to be impossible ("galvanic isolation" is supposed to preclude this), and even if theoretically possible it should still be incredibly difficult to even create such a situation, but in my mind this would be the area to look at. And if the network DAC is running on Power Over Ethernet (POE) -- which I've never seen but is theoretically possible -- I'd be even more concerned, since the twisted pairs with power should be able to pick up some noise, and since noise in the power is something that we want to avoid, the concern here is pretty obvious.
Definitely not! I've worked with SWIFT (European financial transfers), NYSE, NASD, LSX, FSX, etc., and even one bit of information being wrong could cost them millions of dollars (and possibly more). One of my banking customers on (the actual) Wall Street had a back hoe accidentally sever not one but both of their "redundant HA fiber" cables, and there was significant pain from that.
Like I said, on a modern network, you can transfer an entire hour of CD quality music (e.g. FLAC, or just the entire CD image as-is as an ISO) in 1/2 a second, 100% bit perfect guaranteed.