well, I've received a new surface 4 days ago and applied all the updates and everything. sound is very good now.
I've pluged and unpluged headphones several times, played extensive games while charging the surface so as to generate as much heat as possible and the sound is still good. no crackling, both speakers at same volume and general volume is good.
I still have 5 days before I have to return my old surface (advanced exchange) so I'll keep doing extensive testing until then.
So far, all is good :)
If others want to return theirs, go to myservice.surface.com