After defeating Solidus Snake at the end of the game, he falls to his death from the Federal Hall building. Just before he falls however, if you look closely, you can see his lips move slightly in the brief moments that they are not covered by the collar of his exoskeleton. By replacing Solidus' model with Solid Snake's (who shares most of his facial features) through mods, it reveals that Solidus was actually saying one last message to Raiden. However, the dialogue during this moment is still muted, so his last words remain unknown.