After upgrading to Flash 10 sound stopped working. Installing the flashplugin-nonfree-extrasound package solved that problem for me:

aptitude install flashplugin-nonfree-extrasound

If your using pulse audio, install the flashplugin-nonfree-pulse package. It doesn’t look like the Debian flash plug-in suggests either package:

apt-cache show flashplugin-nonfree|more
Package: flashplugin-nonfree
Priority: optional
Section: contrib/web
…
Suggests: iceweasel, konqueror-nsplugins, x-ttcidfont-conf, msttcorefonts, ttf-dejavu, ttf-xfree86-nonfree, xfs (>= 1:1.0.1-5)