ok more details....when i play i cant hear nothing ive tried different cables and plu ins and it still wont work....everything works fine but the sound
Modded means modified by means of a Modchip installed inside the gamecube or alternativly modified by flashing the bios chip which is already inside. This second option is often called "softmodding" the most common reason for modding is to allow you to play burnt copies of game's.
Its not modded then, dont worry about it. you said that you tried a differant AV cable? It sounds as if the cube is actually damaged, have you looked for a repair person in your area?