ADA995 Humm

If it's done it since new, it would suggest it has a bad power supply.  It's probably out of warranty now (unless you purchased an extended warranty).  However, there are some things you can do to try and isolate what is causing the hum.

Does the speakers hum, by themselves - not connected to the PC?  Disconnect the speakers from the PC and pull the cable back to the speakers so there is no chance of some AC source causing interference and then power on the speakers and see if it hums.  If they still hum, it's the speaker power supply.

If the speakers do not hum, disconnected, route the cable back to the PC the way you originally had it routed, but do not plug it in to the PC's sound card.  Then see if it hums.  If it does, it's the interconnect cable and it's routing, probably near an AC power cord or source - rerouite the cable and see if it fixes it.

If it doesn't hum in the previous step, reconnect the cable and make sure the plug is fully seated in the sound card as only partially inserting the plug can cause hum.  If it still hums with everything conmected back up, it's probably an issue with the sound card.  The next step would be to power off the PC, open the case and "reseat" the sound card.  "Reseat" means to unpug the card from the expansion chassis connector and then plug the card back in, making sure it's straight and fully seated.  When you handle cards, only handle them by the edges as there are static sensitive components on the card and if you touch the components or the "land" (Printed circuit side) you could damage the card. 

If reseating the card doesn't fix it, the sound card is probably causing the hum and the only alternative is to replace the sound card.