If you drag a file over an icon and the icon doesn’t light up (i.e. the target application doesn’t recognize the dragged file as being of a supported type), try again — but this time, hold down the Ctrl key while you drag. This will force the target to try and open the file, even if it isn’t a registered handler for that filetype.
For example, if you register SoundPlay as the default player for MP3 audio, you will be unable to drag a .mp3 onto CL-Amp to play it with that app. Holding down the control key while dragging the file tells Tracker to bypass the default filetype association.
Of course, this won’t work if you drop a file of a type unrecognized by the target app; for example, dropping a GIF image onto SoundPlay would only cause problems (because SoundPlay would try to open the GIF as an audio file).