But that's cosmetics right now. Have you tried playing them seperately in a clean, simple project? Not all formats are playable/convertable. If they do play by simply adding them to a fresh project with nothing else than the audio plugin and 1 event like 'On start of layout' -> Play "whatever", then you should share the capx of your game project, as there might be something else making problems.
As the manual says, the only difference between the music folder and the sounds folder is that files from the music folder are streamed, while files from the sounds folder are buffered completely in RAM. If you're using node-webkit, the loading of the files from the sound folder will be quicker, but the decrompression will take the same amount of time. And the need for enough RAM stays the same, too. Decompressed stereo audio will use 10 MB/m. Also, the audio will be kept in RAM until the layout ends. So be careful when using long music files as sounds. But basically, yes, it's possible.