The voiceover sounds can be changed and/or removed but you cannot add one more.

Calhoun_Rx.ogg are random sounds used while the promo code for Sgt Calhoun is enabled.
Ralph_Rx.ogg are random sounds used while the promo code for Ralph is enabled.

Calhoun_Start.ogg is a sound used when you activate Sgt Calhoun promo code.
Ralph_Star.ogg is a sound used when you activate Ralph promo code.

WinSoundX.ogg are random sounds used when you win a level.