Where did you get the french voice file from? Did you generate it with the Rockbox Utility? If you did - what options did you chose? Which Rockbox version is the Ipod running?
As far as I know, the Rockbox Utility is able to create voice files with the help of a text-to-speech engine installed on your computer (but I admit I never tried myself yet.) Please give more details as it is almost impossible to help you without more information.
If you first want to try out the voice feature and could live with English for testing, you could download the right file from the
Daily Builds and Voices page. Make sure you use the correct one for the Ipod Video, others will not work. You would also need to chose English as language then but it would be a start so you can see how things work.
P.S. Just some other thoughts: first you could check whether voice is enabled. It is enabled by default but maybe you disabled it accidentally? It is also important that the voice file is made for the (almost) same build you are using - you could check this. Additionally, the
Voice chapter in the manual could give some ideas - for further help we would need more information.