Using I-Sonic as a DVD player

Overlook75

I recently purchased an I-Sonic and have been pleased with the performance as a CD player but have had a problem using it as a DVD player. It seems to start out ok but as the movie continues a hissing increases in the audio. I have the I-Sonic connected to my TV with an S-Video cable... Should I not be using this as a DVD player?.... Could it be a problem with my I-Sonic?....