An excellent thread Mykuhl. I have considered exactly that same question many times.
When you think, or dream in bed, you imagine all sorts of conversations going on, or at least I do.
I don't necessarily "hear" someone speak with a specific sound or accent, but within me I "know" its them because of other non specific signals. I might hear someone address me from behind, and in my dream I know its my wife, or a teacher from my first school. The former voice would obviously be familiar, the latter, being from 60 years ago, not a chance. But within me, I know who both are.
I believe this is because in the realm of the spirit, sound is not the medium. Word sounds are just a code system to transfer our inner thoughts or concepts. The real language is the concept itself, from which the word springs. When we enter the heavenly realm, I believe that we will converse by thought concepts addressed one to the other.
I believe that this is what happens right now when we hear God speak. There are times when I have heard God speak with great clarity. Then when I try to write it down, I find there is a conflict within me as to what the exact words were. I then realise that there were several phrases overlaying each other spoken simultaneously, rather like several overhead projector transparencies on top of each other. They all convey the vision separately, but amplify it together. You can see this with some of the prophetic scenes in the bible.
Back to your inner thoughts. Demons merely have to convey concepts direct to our inner man and it is we who label them as ours because they are so like our thoughts because they are communicated at thought level. Being subtle, Satan is quite able to match thoughts, just slightly off track but still sounding very reasonable. Then once we start an inner conversation going with those thoughts, I believe that he, or his demons are able to clearly read our responses because we have established the lines of communication.
I hate the thought that he has such access, but that is the only logical conclusion that matches experience. It also ties in with the scripture that has already been quoted.
2Cor10v3For though we live in the world, we do not wage war as the world does. 4The weapons we fight with are not the weapons of the world. On the contrary, they have divine power to demolish strongholds. 5We demolish arguments and every pretension that sets itself up against the knowledge of God, and we take captive every thought to make it obedient to Christ.
Those thoughts that we take captive are not therefore necessarily our own thoughts, even though they float around our heads and keep repeating themselves. I have many times treated them as demonic, cast them out and seen them silenced.
If demons didn't have the ability to commune with us in such ways, there would be no need to issue warnings about doctrines of demons would there?