Behind That Gas Mask?

After killing more than 2 enemies in 20 seconds

• [Evil Laughter] (Several Variations)

Kill assist

• "Mmph mphna mprh." (Possible Translation- "Thanks for the help!")

Domination of a single opponent

• [Mirthful Laughter]

Domination of two opponents

• [Gleeful Laughter]

Domination of three or more opponents

• [Maniacal Laughter]

Revenge kill

• [Satisfied Chuckle]

Teleportation

• "Mmphn nhya." (Possible Translation- "Thank you.")
• "Mmphn frphha herrpha." (Possible Translation- "Thanks for the teleport!")

Healed by Medic

• "Mmphn nhya." (Possible Translation- "Thank you.")
• "Mhnk nhya mgh." (Possible Translation- "Thank you, man.")

Capturing Intelligence/Control point

• "Mmphpry!" (Possible Translation- "Victory!")
• "Mmmphya harrgh mrgha hrghgph!" (Possible Translation-?)
• "Hurrururururu!" (Possible Translation-?)

Defense

• [Satisfied Chuckle]

Match start

• "Hudda hudda huuh!" (Possible Translation- "Go! Go! Go!", "Move up! Move up! Move up!")
• "Mmmmmmmrrrrrrrpppghhh!" (Possible Translation- "Attaaaack!", "Gooooooooo!")

Match win

• "Mmphpry!" (Possible Translation- "Victory!") (One Variation)

Match loss/Sudden death

• "Mhhhhoooooo!" (Possible Translation- "Booooooo!")
• "Egh yrgh mghma!" (Possible Translation- "Ah, your momma!")
• "Yrghfrgh!" (Possible Translation- "You fools!")

Match draw

• "Mmmrpgh crpyha drghya!" (Possible Translation- ?)

Set on fire

• "Mmmh, Mmmh!" (Possible Translation- "Fire! Fire!")
• "Mmmrgh!" (Possible Translation- [Pained Whimpering])

Quoted from TF2 Wiki.
More character-specific lines available here.