Career networking dude here (Skywalker sound, Lucas Labs, Pixxar, ILM, etc.) Beware of techno nonsense, which among audiophiles seems to be a primary strategy for separating customers from their money.
Noise on Ethernet cables. I agree with the CCNP guy who commented earlier. Unless, you are designing Ethernet signal processing/encoding silicon, you don't need to worry about it. It's not an analog signal, so what you're worried about doesn't work that way. The asterisk to this is a residential technology called PowerLine, and that's a hard no for audiophiles.
Cable type. Do you run 40GigE in your house? Neither do I. Decisions about cabling are based on the speed of Ethernet you want to support as well as distance. You can google a chart on it, but cat6 will generally support 1GigE, cat6a will support 10GigE and cat8 can do up to 40GigE. Please do make sure when running cables to make sure you are not violating fire codes when running through, for example, plenum spaces.
As far as 'audiophile switch', hang onto your wallet. Residential audio applications are low bandwidth applications that do not require a commercial grade router or switch. What you probably want is something that doesn't use up a lot of power (look for max power load/draw in the specs) or doesn't have fans to make noise. I like the new cisco catalyst 1000 series 8 port switch because its fanless, doesn't draw much power and can even do some simple layer 3 routing. There are other Ethernet switches starting at about the $50 price point which may be fine for non engineering users.
Latency. Your intra-switch latency will be on the other of 3-6usec (microseconds) for a store and forward. Humans, starting with elite drummers, start being able to detect viariations in the timeframe of a few miliseconds. So you're fine.
Wireless. Packetization delay and its associated variability in delay (jitter) can be avoided altogether because you have existing wiring in place.
As far as the decision to put in another switch: Your driving design decision would be to control the length of cable runs (a moot point in a residential home) and avoid needless complexity and hassles in cable runs.
If you've got money to burn, I wouldn't buy fancy cables and routers. I would however consider getting a firewall and put in policies to isolate my IoT/security vlan from my engineering, music, guest and production (kids, wife, TV, etc.) vlans.
