By the sounds of things I wouldn't worry about it too much. Seems like it's just a Soundcloud issue rather than anything to do with your composition. Have you tried downloading the uploaded file from Soundcloud yourself and seeing if you can notice the volume changes? I've listened to the track quite a few times now and I reckon it's pretty good. Keep doing more, evolving your skills, sounds and ideas and don't be so damn self-depreciative!