These are some very surprising videos from youtube.

Croaking horned frog (Ceratophrys ornata):
YouTube - Ornate Pacman Frog croaking!

Very angry Ornate horned frog (Ceratophrys ornata):
YouTube - Angry Frog

Another angry horned frog (Same frog that is croaking in the first video):
YouTube - Ornate Pacman Frog getting defensive!

Another growler (probably Ceratophrys ornata):
YouTube - ONE PISSED OFF PACMAN!!!!