I've never been a big fan of trying to call muley's. You'll hear some chime in with a few things but by and large muleys are not hunted with calls. I've tried a little bit of everything from trying to use elk calls to fawn in distress and I've never called in a muley.
For elk diaphrams are really versitile and make about every sound out there and has a hands free operation which is great for stopping a bull for a shot. I also like to carry an external reed call because they make a much better sounding cow in heat call. I also carry a grunt tube for bugling (mostly just for locating bulls) and use a diaphram with it. I usually carry two hand calls like a hoochie mama and together with either a your reed call or a diaphram you can sound like a herd. All these calls used at the right time can be very effective.
The real trick besides learn how to blow these calls is learning the elk language and knowing what to say and when to say it.
Try out this site for some awesome calling videos on learning how to call:
http://www.elknut.com/