I had problems with speaking for a while. It wasn't that I couldn't, but I just had this strange disinclination to when I was lucid. I think it was because I knew I was "in my own head," and no one would hear me anyway. I think that's the same reason why I tend to ignore and not care about the other dream characters. They're all me, anyway.

Anyway, try to look at this problem in terms of a case by case basis. If you expect not to be able to talk in the general sense, you very well may cause yourself not to be able to talk. Believe me--you can. It's just that it was that one dream. Try again next time.