The reason angels and spirits are so intimately united to us that it seems to them as though our characteristics were their own is that there is such an intimate union within us of the spiritual and the natural worlds that they are virtually one. However, because we have separated ourselves from heaven, the Lord has provided that there should be angels and spirits with each of us and that we should be governed by the Lord through them. This is the reason there is such an intimate union.
It would have been different if we had not separated ourselves, because then we could have been governed by the Lord through a general inflow from heaven without having spirits and angels assigned to us. There will be more on this later, though, where we describe how heaven is united to us.
When angels and spirits talk with us, it sounds just as audible as when we talk with each other, but it is not audible to people who are nearby, only to ourselves. This is because the speech of an angel or spirit flows first into our thought and then by an inner route into our organ of hearing so that it activates it from within. Our speech with each other flows first into the air and comes to our organ of hearing and activates it by an outward route. We can see from this that the speech of an angel or spirit with us is heard within us, and that since it activates our hearing mechanism just as much [as our speech with each other does], it is just as audible.
The fact that the speech of an angel or spirit flows down into the ear from within has been made clear to me from the way it flowed into my tongue as well and made it tremble slightly, though not with the actual motion involved when we are articulating the sounds of speech in the formation of words.
from Heaven and Hell, Sections 247, 248