While the previous posters have given information/opinions about putting a sound decoder into the existing chassis, I'm going to suggest something a little different:
Look for a clearance-priced (or used) P2K SD7 with sound, and simply swap out the chassis. You can then sell the "old" chassis and "wrong" body shell to recover some of your expense.
I've converted three P2K GP's and two P1K F3A's to sound using this method, and with some judicial shopping, the end cost per conversion was the same or less than a good sound decoder and speaker.
Depending on the configurations of the specific donor/recipient loco pair, some minor mods may be required, such as shortening a light bar, etc. But I don't know that I spent more than an hour or so on any of the conversions.
Mj-
The engine you have is a good choice for a first time installation. It's easy and enough of a challenge to put some skills to use
You have a choice between three decoders; LokSound, Tsunami, and QSI Revolution/Titan
All three have their pros and cons.
Tsunami good sound, less than average motor control [smooth starts & stops], no software needed for programming.
LokSound good sound, good motor control, software/hardware needed for programming and sound files
QSI Revolution/Titan good sound, outstanding motor control, software needed for programming; perhaps the best of the bunch.
For an install specific to your engine:
-you can modify the frame to accomadate two mini-oval speakers using a hack saw to cut the metal at each end of the frame.
-You'll also have to shave some of the frame above the motor to accomadate an 'Atlas' style board. This can be done with a file/belt sander/dremel tool
An outstanding dealer in DCC products and customer suppport is http://www.tonystrains.com/
Check out http://groups.yahoo.com/group/RPMclinics-RickBell/ for further information regarding tips and sound installations.
Erik Fiske
I couldn't fix your brakes, so I made your horn louder
I like Tsunamis. If the loco has a light board, chances are it can be replaced with a Tsunami sound decoder. The larger problem is going to be where to fit the speaker with its enclosure. I usually put them in the cab.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
The first thing you need to do is open up your model and see if there's space for a decoder and whether there's room for a speaker and enclosure.
Here's some information about putting a motor control decoder into one of those models. A Google search turns up a lot of information about your model, but no one mentions putting sound into it..
http://home.roadrunner.com/~mrwithdcc/llsd7.html
Im looking for some recommendations for a drop-in (not plug and play) sound decder for an older P2K SD7. Which one's do you think are the best?