I glanced over the title of this topic and I thought it said 'Sneaker problem'
As in which sneaker to put on first? The left or the right? (Which made me think of 'Friday'; gotta decide which seat to take)
Forgive my ramblings
Anyway, this 'underwater sound' kind a sounds familiar. Like there's a blanket over the speaker or something? I had this with a speaker years ago, and I can't remember what it was. THough I don't think it's loose contacts, then you'd probably just get crackling noises.
I'm going with DarkFish's view on this; the problem could be inside the speaker. I'd also try hooking it up to the other channel as Sharkbit said, but I think it's inside the speaker itself, maybe the coil is damaged?