Gold plated connectors do offer the advantage they don't corrode so they will last longer. Professional institutions may use higher quality cabling than the super cheapo hifi stuff but then that's because it'll be mission critical, may be installed in hard to reach places permanently so they don't want stuff going faulty.
They might pay up to £3 a meter for some cables!
But yeah, digital cables can be any old nonsense as long as it's been reasonably well assembled. Not sure what engineers approach to building cable such as HDMI but I assume they are a little too complicated to solder your own? Although I guess in areas such as broadcast HDMI perhaps isn't used?
Speaker cable also doesn't matter. The signal is SO high voltage that any interference etc picked up is likely to be WAY, WAY below that of the desired signal. Like you would need serious scientific equipment to detect it.
'Line level' cable also doesn't matter too much. Voltage is significantly lower than speaker cable, but still sufficently high for any interference to be largely inperceptable. Apart from heavy stuff like RF interference, e.g. your mobile phone going pippety pip pippety pip through your hifi, but then I find that kind of interference useful as a heads up that my phone is about to go off
What always makes me laugh is that hifi buff 'audiophile' types spend hundreds of pounds a meter on UNBALANCED PHONO (rca) CABLES to connect all that overpriced gear together! Seriously, a balanced jack cable or better still 3-pin XLR cable will have none of the potential issues a single piece of wire phono cable can have. Then they spend £500 on a power cable ffs. Morons!
Microphone cable it is worth spending a little money on. Like £3 a meter. Signals coming from a mic will be VERY low compared to line level and noise can be an issue if using cheap poorly shielded cable.
