Yes you can add custom speech to the speech menu, but I warn you all it means is extra downloads for your players that neccessary.
Here is an example:
If you add this to the bottom of your speech list, these voices will work if the sound files are in the appropriate folder.
Code:
"Last Chance" "spk \'ambience/0Soup0_lastchance\'" "cd" "u"
"Ha Ha (Nelson)" "spk \'ambience/0Soup0_haha\'" "cd" "u"
"Fatality" "spk \'ambience/dead_mk2\'" "cd" "u"
"Stop, Police" "spk \'ambience/dead_police\'" "cd" "u"
"No, No, No" "spk \'ambience/0Soup0_apunono\'" "cd" "u"
"Idiot" "spk \'ambience/idiot\'" "cd" "u"
"Matrix" "spk \'ambience/matrix\'" "cd" "u"
"Ticking" "spk \'cs_wolfenstein/ticking\'" "cd" "u"
"Djeyl Laugh" "spk \'djeyl/laugh\'" "cd" "u"
"Mario" "spk \'mario/smb31\'" "cd" "u"
"Bend Over" "spk \'player/bendover\'" "cd" "u"
"Come Again" "spk \'player/comeagain\'" "cd" "u"
"Dohoo" "spk \'player/dohoo\'" "cd" "u"
"Doomed" "spk \'player/doomed\'" "cd" "u"
"Numnuts" "spk \'player/numnuts\'" "cd" "u"
"Terminator" "spk \'superheromod/terminator\'" "cd" "u"
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