[Alessio Degani]
  With my sound card, is impossible to set jack to run
at 8 kHz. 
[...]
  What can I do to test my algorithm at 8 kHz in
realtime using jack? 
Jack does not support multirate processing, so you'll have to do the
down- and upsampling in your client.  For a C application I'd look at
libsamplerate first.
Cheers, Tim