A HDMI cable is a digital cable so it doesnt suffer from interferance like analogue cables do, where as a subwoofer cable is analogue and can suffer from interferance. Looping the cable can make it act like an arial picking up static from near by equipment, which is what can cause the humming effects mentioned earlier.
As far a the OPs question, if the 5 meter cable is free then give it a whirl, if you notice any issues then get a shorter one. I have used longer cables before with no noticable issues so it is possible.ÿ