Remember how it spoiled or before it happen?

Maybe you can try turn the volume from 0 to full for few times without playing music, and press many times on and off on the mute button. I experinced M3300 mute button will had slight oxidation inside causing 1 of the channel not working properly or can't fully mute.

Plug contact possibly oxidized, try find alcohol or contact cleaner to clean it, wd-40 works too.
How come your M3300 got 3.5mm to RCA one? Mine don't have wor 

Eh i think should be contact oxidize problem if you didn't do anything harmful to it.....contact cleaner is not contact lens cleaner, it special build for cleaning electronics stuff one......your alternate choice is using video tape spray....not as powerful as contact cleaner but cheaper....around RM12 or lower for a spray can.

Spray on a tissue then rub the 3.5mm jack....spray some on the serial port that connected with the remote one and leave it until dry before plugging in. If that step won't help, spam the mute button like arcade joystick 

for oxidation problem you can use wd-40 to clean the oxide out

Spray on tissue and rub the jack clean then a dry tissue to rub it dry 

that means not oxidised lo...
something must have snapped or broken

time to send it back for warranty or get a new one

Sad case....did you tried spamming mute button and turning the volume up and down like mad?
Any other source to test it out?

If all above done liao still not working then really need to prepare for a new set.

ehehe... time to save up for a better one

Casperwinde can donate me your destroyed unit? i want to be a proud owner of a real edifier, i don't own one and i love broken stuff =)

Go for edifier bookshelf speaker 

I don't actually understand what you mean 

Hmm...maybe i get what he mean...

Cable can be 1st thing if the cable upgrade is cheaper than other upgrades. If the DIYer found that cable does not help much then it is up to them....and that make sense cable might not anything to them.