Well, I've a similar problem with the original AV cable on my Duo. Or it could be just the CD player. The sound jumps in volume when I play CD games. It's not the CD's cause I tried them on a backup Duo and they worked fine. Sometimes the volue is normal, other times is just low. Don't have this problem with HuCards.
Should also mention my AV cable fits loosly as well on this old Duo. Probably from all the unplugging/plugging.