It's really hard to generalize with speaker cable. Speaker and amp electrical characteristics can vary a lot and best SC can vary a lot too. If you got 5N silver that can sound really nice, but 12g wire isn't intended to be bent repeatedly so over time the wire will crack. IMO, you're best off using multiple runs of smaller gauge wire, maybe 24g or so. Or see what you can get Neotech UPOCC silver/teflon for. It's 6N+ and far better vs regular silver, but it can be pricey.
I would recommend to get some inert gas, maybe just something used to fill wine bottles, and flush as much air out of the teflon tubes and seal the ends to prevent corrosion. Sealing the ends isn't easy though, there are some primers that allow cyanoacrilate to stick to some degree, but the best adhesives are very expensive and not for sale to the public.
Another option would be to use enamel to paint the wire and thus prevent corrosion, this is how magnet wire is insulated.