I can't really find anything with real life exemples and values, and formulas often get too complicated for a dude who stopped school 15 years ago and math/physics even sooner ^_^.
all I can find talks about high freqs as in radio freqs, but nothing simple or really relevant to my eyes (meaning: that I can understand) in the audio range.
I was thinking about an increase in impedance from shielding like this:
signal in the cable generates magnetic fields that will create a signal in the the shielding material that will itself create back magnetic field that should in the end oppose voltage in the cable in some ways no? I don't know if this super inefficient transfert can matter at all, but as it depends on ... voltage?(I'm guessing it's voltage for magnetic field) ... the changes arent linear and might matter in some ways. I just have no idea about the values of such effect.
also what I really know nothing about is how the shielding material behaves on portable stuff (like IEM cable) where it's etheir not grounded at all, or linked to the circuit ground of the cable itself? I really don't know much about all this, but my little pinky told me proper shielding shouldn't be like that.
and about the cable with a few ohm, again from a very insecure memory, I believe the purpose was to make a little change in signature thanks to the weird impedance/freq caracteristics, to ease up on a given frequency range. I guess it was a situation not unlike the W4 where the signature is almost an exact opposite of the impedance graph and adding resistance tend to make the sound flatter(not taking damping into account).
Not happening. If anything the capacitance between the grounded shielding decreases impedance as the frequency increases due to capacitive reactance between the shield and the signal wires. Your theory about magnetic fields is just being over imaginative, this is not going to happen in an IEM or headphone cable. Though your little pinky is smart enough.