Same thing happened to me 2 years ago. If you don't do the things I'm going to suggest you'll have the smell of curdled milk to deal with also
.
The milk has drained down into the spare tire well in the center of the trunk area. Right now your radio equipment, some of which is UNDER the spare tire, is sitting in a pool of milk.
.
1. Prop up the cargo carpet/cover
2. Remove the spare tire.
3. Remove the battery and audio equipment and let it dry for a period of time.
4. Wipe up all remnants of milk from that area and spray it with a disinfectant so it doesn't smell later.
5. Reinstall all equipment after drying and if it doesn't work, it simply failed on its own
and you may be able to get it replaced under warranty.
It'll take 4 hours or so but it sure beats having your cool X5 smelling like stale yogurt!