As a guy who would tend to use more feminine sounding usernames, I've been "mistaken" as a female before online in text based communications channels. Sometimes I'd let it go for a while, or I'd comment on the issue immediately. Usually when I would comment about it, I would do so in a very succinct and matter of fact way. Something like "Oh, FYI, I'm a guy, not a girl btw...". Usually thats all it would take to correct the issue. Most of the time no questions would be asked, but I'd occasionally get the "Really?" response, and a simple "yes" is all that needs in return. If they ask about your voice, just acknowledge the fact that you have a fairly high voice, but that you are indeed a male. That should be enough to correct the issue. Best of luck.