I hear a lot of producers layer all kinds off weird sounds underneath their main elements, eg. a babies cry, or a woman's voice under a snare, a cats purr under a bass element in music...of course one doesn't just go about layering these sounds...the content must be harmonically compatible...the overtones and their structure must match, making the two elements blend and sound as a whole...What are the tools and methods to this sorcery, how do you go about matching overtones of these elements...do you EQ, pitch the sound up and down, frequency shift them etc...Please feel free to share any insight without minding how "cliché" or underrated the method is.
Your question is a bit related to another question that I just answered, so at the risk of being a bit tangential, I'm posting below both the question and answer. Hope you get something out of it, in any case.
Q: Is the emotional response to sound universal or does the way we perceive and interpret acoustic stimuli vary from one person to another? Do cultural and social factors affect the perception of sound in any way? How about the technology used to project the sound?
A: We have hierarchies of perceiving and interpreting sound, some of which will be very similar for everyone, others that will differ. I call them listening modes, and in the past have used Michel Chion's terms of reduced, causal and semantic, but now prefer the terms shape, source and meaning.
Shape relates to the physical waveform, which will be perceived quite similarly for everyone, unless they have some defect or distortion as with loss of hearing in high frequencies that happens with older people. We can include variables such as volume, pitch, timbre, attack, reverb, etc. as being perceived universally.
Source refers to the origin of the sound, which is given a name or label if it is recognizable. This can be more or less refined based on the cultural context. For example, the sound of a motorcycle is familiar to people around the world, but the kind (Harley, Suzuki, Honda, etc.) might only be distinguished by those who ride motorcycles. Same situation with dog barks (what breed?), children laughing (whose child?), radio news (what station?), etc.
Meaning has to do with the interpretation of the sound for its emotional or informational content. The obvious area of cultural differences is that of spoken language. An American might know that Chinese is being spoken by the appearance of the speaker, but not understand what is being said. The American might think that the Chinese person is angry because of the forceful expression, but in fact they may be just excited about something pleasant that has happened.
When creating a soundtrack for a film or game, it is important to know who the audience will be, because some of the sounds will be specific to one culture but not another. Globalization has smoothed out these differences, but they still exist.
I have different tricks depending on the context, but a good place to start as you mentioned is EQ. Removing what is unnecessary or in the way and emphasizing the qualities. Pitch also goes a long way to help morph sounds together.
For creature sounds, I to use melodyne quite a bit. For example I'll have two distinct sources... one a horse, the other a human vocal roar. I analyze the pitch of both files in melodyne and adjust in order to match. Gives a nice character and helps sell the multiple layers. From there I add other elements that tend to be less "pitch" important. Here are a few examples I created that demonstrate this method:
Another trick is to process the various sounds through the same bus that contains your reverb, eq, compression, distortion, etc. This gives it the same characteristics and helps hold it together as one.
The most important thing is to move your sounds around and see how they fit and interact together. Sometimes it works, sometimes it doesn't.
It's not clear to me if you are talking about this in a musical context or sound design? Some clarification may help.
In sound design Anthropomorphism is using human related sounds as FX for non-human items. For example, a door opening and instead of using a classic creaking sound, a human groan might be used to give a more sinister feel. This can help to underline a theme within the scene. Is this what you mean?
By doing what you think is right, interesting or emotional. While you can read psychology and stuff and theorize, what there really is, when creating, is only you.
All art forms are either self-expression or collective expression (the sum of many individual self-expressions that have communicated with each other).
There are many sound design conventions that humans have learned to recognize. I.e. using certain or certain type of sounds, rhythm/timing, harmonic properties and contrapunction creates the same type of emotional impact every time. Listen to e.g. movie trailers, many of them sound very similar (Why? Because they're effective that way.). Or listen to movie music and the emotions it describes (you'll notice how a certain type of theme or emotion is often described in someways similar type of composition, instrumentation, chords or rhythm/tempo).
To the question of layering, just use your ears to determine what sounds correct and good, and try different things, that's all there is.