I agree, I think they can recognize the difference in a yelp.
But I wonder how important it is? Think about it, everytime we enter the woods to hunt, we make a new, never-heard-before yelp to the local birds. That most often does not spook them if the calling is done in proper context...another subject area...
And gobblers immediately respond to your calling, a yelp from a new and different bird in the area. So like Shad said -- a new tramp had moved in, does a new sounding yelp inpart excitement or curiosity in a turkey? Or do enough birds move into and out of an area during their lifetime for a new yelp to not matter at all, its just another turkey...
How do turkeys respond to a new, never heard before yelp? It may be partly why hens walk away in the morning.