No audio on Leopard after updating to Quicktime 7.6.6 (or was it 10.5.8...)

Question

Subject says it all. In more detail: after having, in quick succession (mistake number 1) updated the OS to 10.5.8 (from 10.5.6) and iTunes to 9.1 (can't recall from what), and lastly Quicktime to 7.6.6 (cannot recall from what), the Quicktime player can no longer access my audio card. However, all other audio apps including browsers, Logic, iTunes, whatever - all can access the audio card (HDSPe from RME). And Quicktime will send audio correctly via USB and to the internal speakers (MacPro early 2008).
My attempts to roll back to QT 7.5.5 produce the same result. Since these three updates all took place in quick succession, and since I didn't have any reason to check the Quicktime Player before a few days later, only then discovering that it didn't work, I cannot know if it was the Quicktime update or even the OS update to 10.5.8 that went wrong.
Have done usual things, like reinstalling (QT, but not the whole OS), trashing relevant prefs files, still the problem does not go away.
Anybody know what this is all about?

Answer 1

Still no fix.
Two things were changed, imemdiately before noticing that Quicktime Player no longer plays .mov files:
1. OS X was updated to 10.5.8, form 10.5.6
2. Quicktime was updated, the version changing from from whatever is installed by default in OS X 10.5.6 to Quicktime 7.6.6
Since then, for instance using this url (http://www.apple.com/quicktime/qtv/specialevent0110/) with several Quicktime movies on the Apple web, the video plays but no audio is heard on my audio card, which works for everything else on the system. Thus DVD movies work, mpg files work, .mov files in applications other than Quicktime Player, all audio works. Playing the clips on the url above works with audio as well but only when audio is sent to the internal speaker in the MacPro, or line out. The audio card is a stock RME HDSPe AES, running to an Aurora 16, from there to the world.
What got broken? Everything indicates it is an Apple software fault...