What I describe here is obviously a bug in the flash plugin for linux
and not a bug in mozilla. However, describing the problem and the solution
here may help a few mozilla users.
The effect is simple: no sound at all in flash. The reason ?
On Debian (unstable) and may be other distributions the esd socket file
has been moved from /tmp/.esd/socket to /var/run/esound/socket.
This change is normally resolved automatically by the dynamic library
libesd.so but the flash plugin is statically linked with an old version
using /tmp/.esd/socket.
A temporay workaround is to create a symlink from /tmp/.esd to
/var/run/esound
ln -s /var/run/esound /tmp/.esd
remark that on debian the user must be added to the 'audio' group to be
able to use ESD.