win 8 upgraded to 8.1 possibly?
In device manager under sound, video and game controllers , right click the first audio driver (which will be the default for the motherboard chip) and select uninstall. Select OK in the warning window.
Reboot and Windows should install the default audio (basic) for the audio chip.
Ebuyer have the responsibility for your guarantee not the manufacturers by the way. So forget PC Specialist and contact Ebuyer again as the retailer has the onus to fix problems within the guaranteed period.
DON'T BE FOBBED OFF. As the seller it's their problem. Refer this issue back to Ebuyer pronto.