Is it very hard? It's the blue box type rambo1...
That would depend on your skill level. If this is your first time, it might be hard. If youve done 20 or so, it will be easier. Have you taken the shell off to see how much space is avaible for the decoder and speaker? Have you looked into what decoder you want to buy? There are several decoders available on the market. I have a Proto 2K BL2 and there appears to be sufficient space under the hood for both the speaker and the decoder. Make sure you follow the instructions concerning the light bulbs, or you will be looking for a new locomotive shell.
My understanding is it's best to remove the weight, clamp it down, and shave or grind out some of the weight towards the back end (long hood end) with a belt sander. Being careful not take off more than you need. Mount the speaker with a baffle facing up through the models roof fans and give it a go.
I'll be attempting this installation myself once I get some track wired.
Take a look at some of the cell phone speakers that are available. These look like they are just right for fitting into a narrow geep hood without much, if any, filing of the weights.
http://www.youtube.com/watch?v=Y2s7NqSBFVc
http://www.digikey.com/product-search/en?v=423&mpart=2403+260+00001
Do some searching, there are several options. These speakers are used in various smart phones, etc.
http://www.sbs4dcc.com/tutorialstipstricks/sugarcubespeakernotes.html
I have not tried these yet but I will be checking them out soon.
Good luck, Ed
I put a Tsunami in this Proto GP9:
The Tsunami is the lavender thing on top. I made my own speaker baffle, which sits over the wheels on the left:
There was significant metal-cutting required to fit this in. I had to cut down the weight under the Tsunami, and remove material for the speaker as well. On the other hand, the engine sounds and runs just great now. It was my first Tsunami, and I've since bought several more.
It takes an iron man to play with a toy iron horse.