flashplugin-nonfree only support Alsa. Thus, extra package required to support OSS4

#aptitude install flashplugin-nonfree-extrasound
#mkdir /usr/lib/oss/lib
#cd /usr/lib/oss/lib
#ln -s /usr/lib/flashplugin-nonfree-extrasound/libflashsupport.so

Restart iceweasel