Put e.g. the character _ as the first letter in the names of the respective video and sound plugins. (K-Meleon / plugins directory). You can see what plugins do what by typing
about:plugins
in K-Meleon. Later when you need the plugins you can just rename the dlls.
Or if you want to do that from inside K-Meleon, you can download the nice Japanese software "Plugin Chooser" from
http://www.asahi-net.or.jp/~rb2t-kmc/index-e.htm
and use a macro to run it from inside K-Meleon (see macros documentation here and your macro.cfg on how to write the macro). That application disables and enables 3rd party plugins in a certain directory and also shows which plugin is used for any MIME type.