First, you need the xmms source from http://www.xmms.org.
Then, install the files here under /usr/src/xmms/Effect/filter_plugin.
cd /usr/src/xmms/Effect/filter_plugin and make.
Copy or symlink .libs/libfilter.so to /usr/lib/xmms/Effect/libfilter.so
Run xmms, and you should see the filter under Effects in the Preferences tab dialog.
Use the Configure button to adjust the squelch threshold to the needs of your input source.  The higher the threshold, the more aggressively the plugin will filter.
 