I just downloaded audacity on windows 7. First was all right, but then I started to have problems with the export of audios: the program exports audio, but when I open it for hear it there are no sounds, I also tried to open it with different programs such as iTunes and Windows Media Player, but I always get the same result.
Which version of Audacity? (look in “Help > About Audacity”)
If you get a problem like that, drag the exported audio back into Audacity and see if it has tall blue waves.
If it does, then the default Windows playback device may have changed, perhaps because you connected a USB device which took over playback: http://manual.audacityteam.org/man/faq_playback.html#usb.