A similar topic had been closed because of a beta release.
I'm using final release and I confirm the problem. I've tried with a brand new profile of Thunderbird, nothing.
I can hear the sound, but the enveloppe at the top stays white and the icon in unity does not indicate the number of new emails like before.
I'm using the 64bits version of the 12.10 release, fresh install, no upgrade.
Cheers