Ninja Gaiden sound problems???

[Game-Junkie]

I've manage to download Ninja Gaiden... but about 5 minutes into the game, the sound starts to fade in and out? I thought I got a bad copy! so the game being to great, I went out and bought a new copy. came home and again in the middle of the first chapter the sound started to fade in and out. It only comes back when I'm fighting foes? Before I put the game inside my machine I turned of the modchip. Is my modchip causing this?