Back in early 2000 I wired my prior home family room for 5.1 sound, running the 12/2 wired under the room via basement and then coming up at each speaker location.
All open space in the basement, no in-wall, just basic 12/2 wire.
I ran out of wire, went to local Lowes and they had a different supplier for the wire, it was 12/2 but thicker outer jacket, anyway I bought a 250ft spool of it.
Fast forward, I used that in my new home family room home theater, and it did oxidize over the years.
(I took these pictures back in Feb-2010)
Guess what, the speaker still work fine, sound fine, and what not.
I've since used most - not all - of the remaining 12/2 wire on that spool for my outdoor HT set-up.
You can clearly see the "green" 12/2 vs the normal 12/2 - both bought circa 2000, hey gotta use it since I own it.
I've since used the remaining 12/2 length of that "bad batch" to extend speaker wires as I relocated some in ceiling 2' x 2' tile speakers in my basement when I got a pool table just a few weeks back.
Just cleaned up the oxidization, soldered them to the existing 14/4 wire there (more leftover wire from the HT project), and it works fine.