You may need to make the HDMI display default in the Sound panel of Windows 10. Also check Device Manager to make sure the driver is working correctly without any Yellow Exclamation point.
Thank you for your answer,
As an audio source, I can only select "Speaker" or "Realtek Digital Output". Unfortunately, nothing changes with Realtek Digital Output as standard. The Device Manager states that the device works without any problems. The graphics card is also recognized as AMD Radeon 200 Series, without yellow Exclamation point. The display on the HDMI port is not detected at all.
I looked again manually on the ATI website for drivers, and found out that there is only one driver for Windows XP for the R9 270x series.
It makes a jump from the R9 280 to the R7 270x. Is this a problem on the ATI website?
Or is there no drivers for Windows 10 for this graphics card ????