Sorry for resurrecting this old thread but it was not locked and I experience similar problem with Opus 10.0.3 with media files and Winamp. The problem is not present in other two file managers I tried (one of them being Windows Explorer).
So the problem is: in Opus when I press enter on selected mp3 files all files are opened sequentially one after the other. Because in Winamp I have option to allow multiple instances disabled (is also default) it will just quickly go through all selected mp3 files (presumably as Opus feeds them to Winamp) and end up playing the last one (while others are "forgotten"). I can only play all selected files in Opus by right clicking on them and selecting Play in Winamp in context menu. Once again, this only happens in Opus.
I would very much appreciate if this could be fixed as I am used to selecting music files and pressing enter to play them this way. 