I performed another test: I installed Kubuntu 11.04 in a new partition. Then I checked the sound. It is possible to hear the sound over the HDMI interface at the speakers of the TV-set or to hear the sound over the external speakers.
Then I upgraded to 11.10. The sound is to hear only over the external speakers but not over the TV-set. The same happens after upgrading to 12.04.
I performed another test: I installed Kubuntu 11.04 in a new partition. Then I checked the sound. It is possible to hear the sound over the HDMI interface at the speakers of the TV-set or to hear the sound over the external speakers.
Then I upgraded to 11.10. The sound is to hear only over the external speakers but not over the TV-set. The same happens after upgrading to 12.04.