Comment # 1 on bug 938898 from Takashi Iwai

The YaST2 test sound does simply "aplay default:XXX", and this won't work for
HDMI/DP as HDMI/DP takes a different device number.  Fixing this isn't trivial,
unfortunately.

For really overriding the default sound, you'd need to define like

pcm.!default "hdmi:1"

in ~/.asoundrc or /etc/asound.conf.

We don't do this because user is supposed to use PulseAudio nowadays and PA can
take care of HDMI/DP output better.  The above is a recipe for old school boys
:)