[300.6(A)]. With a corrosion resistant coating inside and outside.
[300.7(A)]. Use an approved sealing method to prevent the
circulation of the warm air to a colder section.
[300.7(B)]. Yes. Use expansion fittings where necessary.
[300.8]. Yes. Don't run anything but your electrical system in
those. While the speaker wire is probably not going to hurt
(although does add to raceway fill), it's not part of your
electrical system. Where things really become unsafe is where
installers run steam, water, gas, or other kinds of service in
electrical raceway. If you see such an installation, immediately
bring it to the attention of the responsible parties. If they won't
act responsibly enough to correct this, then don't work in that
area. Notify the AHJ so that corrective measures can be enforced
before people start dying.