'Wanking ears' is the expression for the vastly increased sensitivity of one's hearing while they are masturbating. This is a protection system so that no-one will walk in on you while masturbating, which could be an extremely embarrassing experience. Even the slightest footsteps out in the hallway will be picked up by your "wanking ears", and by the time this person comes into the room, should they decide to, you will have had time to close down the pornographic website/video you were watching, and zip up your jeans.
Similar to beer goggles, which is the effect of alcohol on sight, this is the effect of masturbation on auditory awareness.
<Pete> brb, wank
<Dave> isn't your mum in?
<Pete> it's ok, my wanking ears will protect me