How do I use a 15vac output supply for 12vac output?
You can use a resistor, I did this with a fan and it worked. you have to measure how much current the system use (I) at 12vac then you can add a resistor with 3/I ohms value. you have to also use a resistor which is rated for at least I*3* 1.5 power. otherwise it would get too hot. It's not an efficient way but it works. Your system needs 10w that means it needs 10/(12) which is less than the current your supply is rated for. You need to do a little calculations, but I guess a 10w 3ohms resistor could work. please let me know in the comments if you need more help with the calculations so i would edit the answer with more details.