Armouredank: Yes, it is likely not working because your sounds are MP3.
As you can see in the manual article about the audio object, sound format is supposed to be ogg and m4a.
Depending on your OS the conversion can be automatic, but MP3 is such a legal mess (read 1, 2, read about the HTML5 audio formats : 1, 2) that it was not recommended by Scirra.
Moreover, prefer importing music into the "Music" folder, not the "Sound" folder unless you know what you are doing (which, no offence, doesn't look like the case at the moment). Again, please refer to the manual (this article in this case).
Inverting is done by right-clicking a condition and selecting "Invert" in the appearing contextual menu.