I think some people without DID have internal voices; we all have aspects of ourselves. My psychologist describes how a person might seem different when at work than they are with their family and again, different than with their friends. Different aspects of a person's personality are expressed in different situations.
With DID, it is more extreme where the parts are more separate. Sometimes they can do things that another part isn't aware of. Some people with DID have voices and others aren't aware of their parts. But there are some other symptoms which signify that DID might be the diagnosis. Some people may not realise there is anything amiss except that they lose periods of time or find evidence that they have done things they don't remember doing.
I didn't tell my psychologist things that were an issue for me for fear of not being believed or of being locked up but now that I am reading about the symptoms of DID, I realise a lot of the things are a common feature and if I'd just been honest and expressed them earlier, maybe it would've helped the psychologist to figure me out quicker. But then in hindsight I realise they figured it out a long time before they decided to tell me explicitly what they thought it was so it didn't matter too much anyway.
These parts were formed in order for me to protect myself and function in an environment where it would have been difficult to tolerate some of the emotions that experiences provoked. Nowadays, life would be much easier if the parts were more in unity, so it's not useful anymore.
I am only one alter in the whole of Candycan and most of the other parts do not communicate very well at all. We are lucky to have this part who can also help some of the other parts to express themselves, but not always.
I dont know if that makes sense to you!