Currently Opera uses your Windows speaker settings for 'speaker fill'. So if you have a stereo audio source it will only play on your front speakers if thats what Windows is set to do. If Windows is set to clone the signal to your rear speakers than that is what Opera will do. The location of this setting can very depending on your sound hardware but here is where I find it on my machine: Right click speaker icon in notification area > Playback devices > Double click your playback device > Enhancements > Enable speaker fill. This will not effect true surround playback. If you play a 5.1 signal with this setting it will sound just like it should.
Here is a way to test 5.1 playback in Opera:
I am not very technically minded but if it's a setting with Windows then how come Opera 1 does it and Opera 2 can't? Is the speaker fill setting automatic for Opera 1 only or something like that? (I couldn't find the speaker fill option for my audio either)