You can try a different 3.5mm cable as some cables can have hiss if even one pole is not connecting well, however if the bar 9 has just been purchased, and you're using the provided cables, I would recommend going back to retailer for a replacement so soon after purchase.
If still within 30 days, you can ask for replacement if a fault develops.
After 30 days they can ask that it be repaired instead.
