In Europe sockets are designated with NEM numbers. An NEM652 is an eight pin, an NEM651 is a six pin decoder.
An NEM 658 is a whole family of decoder interfaces and their pin numbers vary from 8/12/16/22. They are commonly called a PluX decoder with the number of pins after the PluX.
There is also MTC by Märklin, but most of these you won't have to worry about with North American equipment. Those are usually a 6 or 8 pin socket. Some are more advanced though with a 21 pin socket.
Count the number of holes in the socket and that's the decoder you need. Any brand will do, Some sound decoders already come with the speaker attached to leads, and some European locomotives have solder pads right on the CB to solder a cube speaker or leads.
You will want to make certain you get a decoder with the correct sound file loaded for your locomotive. You don't want GP-35 sounds for an SD-70 locomotive.
I use mostly ESU equipped locomotives as they have a vast sound library of locomotives for which they have files to load to the decoder. I try to buy sound equipped locomotives to begin with and avoid this hassle, but some locomotives were never offered with sound.
It depends on how old the engine is.
Newer ones may have 8 pin plugs for decoders, but older ones may not.
Each engine has to be evaluated individually, to determine what needs to be done to it.
Show us pictures.
"Only one of the GP's has a speaker hole cut in the metal chassis"
Again, depends on how old the engine is, and the production run.
A later production run probably has the speaker opening.
Older, perhaps not.
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