Well, I don't think the game supports .mp3 files easily. The game really focuses on using .wav and .ogg files. You can get an .mp3 to .ogg converter pretty easily online. Try
www.download.com and search for a converter.
The building, I believe you can set the cell-size of the unit, so that it requires more space. I don't think you can designate a specicfic spot for the unit.
The elephant looks very nice though. With textures, you won't be able to notice any/many of the blocks, but if you think it looks a little crummy, then you might want to try and fill it in a bit, but it looks pretty nice.
By the way, do you think you could make an elephant rider, where one guy might have a bow and shoot arrows, or one elephant rider has the jousting pole from the horseman, where he stabs people
good luck