Permanent 50hz

If there is nothing connected to the speaker except power cord, and we are assuming it's a powered speaker, (no RCA or XLR signal cables) -- if you are still getting 50hz buzz, then I agree with Eric that the power supply is failing (probably a bad capacitor).  The amp/ps board in the speaker likely needs to be replaced.

It actually sounds like a minor ground loop hum.  If you have to get within 5cm of the speaker to actually hear it, then don't worry about it.  My system has the same thing.  Sometimes this will be very noticeable in systems and people can have a very hard time getting rid of ground loop humm.  Humbuster may or may not fix this.  Sometimes, you'll need to get the Granite Ground Zero to resolve this ($500+)