I use Skype for international meetings and messaging. I've had problems in the past with people not being able to hear me. I can't remember the exact steps in Windows (7 at least, never used Skype with 8 - I hate 8), but it had to do with one of the settings for the microphone not being correct in Windows itself (Control Panel->Sounds).
Other than that, like others, sometimes closing it and re-opening it will help.
That's assuming it isn't a problem with your mic itself.
And I agree that Skype went downhill when MS bought it, I was quite dismayed. Lot of lost line quality and weird things happening here and there. It has gotten quite a bit better, but now every time I get an update I have a weird issue with the login screen.
I also use Vonage quite a bit. I have 0 problems with Vonage (aside from internet issues here and there which I haven't had in quite a while). If I were living in the States, I'm not even sure if I'd buy a regular phone service. Only problem is you need to bring it from home...can't get it here.