But seriously, folks...
No, "Erosion" is not a plug-in. I was being sarcastic (though I prefer the euphamism "cheeky".)
Seriously, it's just that there are so many things that are not undertandable about the question that I'm not sure how else to respond to it. Neither is John from Massive Mastering, who is one of the most experienced and knowledgable guys on this board. "Peaks and valleys in vocals or songs" still doesn't explain just what you are trying to affect. It's like asking your doctor, "Other than asprin, what other drugs are there to get rid of peaks and valleys in my body?"
If you are looking to tame changes in overall volume, then you might look at compressors and limiters, of which there are hundreds of plugins included with most audio editing software and available as aftermarket plugins. Another solution to this problem is to use "level automation" in your editing software. A third technique is to manually edit extreme peaks and valleys (in this case, "transients") in the waveform.
If you are looking to smooth out peaks and valleys in the frequency response of your system or your content, then you have parametric and graphic equalizers for that. Again, there are plenty of stock EQ plugins included with most editors, and a galaxy full of aftermarket plugins available.
If you're looking to do both at the same time, there are multi-band compressors for that, and there are many MBC plugins available on the Internet.
If you're looking for junk food like HarBal to try and do your job for you, there are apps like Steinberg's FreeFile and the independant Volume Balancer app and others. But they all pretty much suck.
G.