Try http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Regular_interval_audio_splitter.
If you are already using that, and Undoing and Redoing that effect with different silence length does not work for you, try the Extend Silences plugin (if you want to increase the silence, which I guess you might).
See here for how to install Nyquist plugins: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#install.
Gale