How to retrieve irssi's variable into bash command

Hi,

I use beep_beep.pl script to launch personalised command when someone 
highlites me.
Some weeks ago I discover festival speech program so I have set beep_cmd 
like this :

/set beep_cmd echo "New incomming message" | festival --tts &

I want now to enhance this command via Irssi's variables 
(http://irssi.org/documentation/special_vars) so I can have some 
information about who or from which channel I'm highlited.
I've tried /set beep_cmd echo "New incomming message from $C" | festival 
--tts &
but it doesn't work. Same behaviour with $$C / ${C} and that "normal" as 
outside irssi theses vars are not sets. (Output is funny :P)

Any Ideas on how to do it ?

Thx a lot :)