I want to see a peer reviewed, scientific report done on this off-gassing of outdoor rated speakers in hot environments.
I'm sure if measured, everything in a Sauna gives off a little bit of something when heated. Doesn't mean it's enough to cause issues or anything that is dangerous.
Until I see a study or report I trust, I call BS on this!

As far as speakers that can take high heat and last in a hot environment - Sonance has the "Visual Experience Extreme" speakers that are rated for tough environments. Including temps up to 194°F. The spec sheet has a list of ASTM standards it passes.