Hi Jiri,
As mentioned at the beginning, this bug was initially created as a clone of Bug #621082, which is a bug on RHEL6. So not a regression from 5.5.
Created attachment 454987[details]
Install guest: "Enable audio: False"
As defualt, all guests (include rhel and windows) will not install audio device on both rhel5.5 and rhel5.6, just like the picture shows in the attach fiel. So not sure if it's a normal feature or a bug.