@Melissa,
What kind of game is it? The player perspective will definitely influence how you approach the skeleton audio. I assume as it is for mobile it will either be isometric, top-down or side scrolling?
Secondly, is it real-time or turn based (like the might and magic games, for example?) or something else entirely? This needs to be considered, as a real-time game will require much more variety to prevent repetition.
Take a look at other games that have great skeleton sound design. The Diablo series, Skyrim, Dungeon Keeper...
I would start with the footsteps and work up from there (literally). Are the skeletons wearing boots? If not work on a collection of good clackety, rattley, [insert made up boney word here] footstep samples. Regardless of how sophisticated the coding is, you should consider incorporating all movement foley into these footsteps and have the "tail" of the sound overlap with the start of the next footstep, giving a nice, constant sound as he/they move.
Grab as many things as you can that have the kind of sound you need. Wooden sticks, gravel, actual bones (leftover KFC! Although you may wish to wash it first...). Try as many things as possible until something sounds right. This will more than likely need quite a lot of EQ to get the tonality right, but it depends how lucky you are with the source sounds.
Does the skeleton have a voice? If not use the same ideas you used for footsteps and try to create vocalisations with the bone sounds. For example, a fast rattling sound resonating from inside the skull could work well for pain or attack vocal sounds.
Does he wear any plate or chainmail armour? Adding this as another layer to the bone foley will help it feel a little more alive than just the bone sounds.
For your joint idea, you can't go wrong with a selection of rusty old hinges. Just be careful not to go overboard. less can sound like much more if you strike the right balance.
~Dan~
