>> Hmm, I don't find that in Debian Sid repository. Search for 
>> "libflashsupport" only finds flashplugin-nonfree-extrasound:i386, 

>> which only connects Flash to Esound or OSS before falling back to ALSA.
>>
>> Is it available from somewhere else?

It doesn't seem to have a web page but you can grab the source from here:

git clone git://repo.or.cz/libflashsupport-jack.git