I'm curious about this too, as even though I haven't used Construct yet I've been meaning to try it out.
Looking at the XAudio2 object it seems like music and sound effects are treated separately, even though it seems like you could play music through a sound effect channel if the music was in .ogg format. You can check if a sound effect is playing, but I don't see how you can tell explicitly if the music stream is playing...it seems like the conditions and properties of XAudio are available only for channels (for which you
could check if a file is playing, etc.), not the music stream, or am I not understanding that correctly?