I am sure someone else has had this issue, but all of the other topics I have looked through did not help me. When I had a Windows machine I used Audacity and absolutely loved it. I used it all the time. I now use a Mac OS X Version 10.6.8. I downloaded Audacity and made the mistake of automatically running it from the DMG file. So I moved it into a folder I created in Applications and went to open it again. It opens fine and I added an mp3 and went to play and NO SOUND. So, to double check if its the program I opened iTunes and NO SOUND. Even when I turn my volume up and down my Mac will sound off a beep every notch I turn it up or down. I have removed the program completely from my machine, ran a system update, and restarted the computer twice and nothing is working. What should I do???
So the volume control beeps do work?
Apple (upper left) > System Preferences > Hardware > Sound > Output and make sure Internal Speakers (or whatever your normal sound is) is selected and the volume slider is turned up. Some applications like iTunes have their own volume sliders and you need to make sure those are up, too.
This can also get messed up if you plug in USB sound devices like external speakers or a USB microphone.
Koz
Thank you for the quick reply. I must clarify the volume control beeps are also not working. No sound whatsoever from any program on my Mac. I checked what you mentioned and all is well there. Still no sound.
OKAY! It is NOW working. This is what I did after I completely deleted the program. I went back to this site http://audacityteam.org/download/mac and downloaded the ZIP file. Before running the zip file I created a folder under ‘Applications’ and named it ‘Audacity’. I dragged the zip file to that folder and then unzipped it to that folder. I opened the program and opened an mp3. When I played the mp3 there was no sound. Then I plugged in a headset and the sound was playing on the headset. Then I unplugged the headset and the sound is working from my internal speakers. I am not sure if deleting the program and reinstalling it makes a difference or if the dmg or zip file differs OR if it was simply plugging in a headset and removing it. But regardless it is now working. 
I had exactly the same problem:
- I am on OSX 10.8.5
- I downloaded Audacity and ran it directly from the DMG
- Audio stopped working system wide
- Plugging in an external USB speaker let audio work again
- After unplugging the speaker, audio still works.
It is not likely to be relevant to your problem but please do not run Audacity from the DMG or you will not be able to save projects or export files without making folder changes, and you will not be able to use most of the Audacity plug-ins.
Install Audacity thus:
- Double-click the downloaded DMG to mount it
- Then to install Audacity, copy the “Audacity” folder from the newly mounted DMG to /Applications or any other location of your choosing
- Eject the DMG using Finder, then to start Audacity, double-click Audacity.app in the folder to which you installed Audacity.
As Koz said, if you plugged in a USB recording device because that is what you wanted to use Audacity for, then the USB recording device is the problem.
If you unplug a headset and it does not release the clip properly then the Mac will still think the headset is there and still cut off the built-in audio.
Gale
The DMG is the abbreviation for “Disk Image.” It’s the tight, tidy package that contains all the pieces needed to make a proper Audacity installation. Unfortunately, Audacity will try to run directly from the image if you tell it to.
Without the installation step, Audacity does not fully “know” about the computer running it. This can cause confusing and awkward problems as Audacity tries to put files in places they should not be and look for files that aren’t there.
“My Audacity is insane. Please help me!”
“Did you install Audacity or are you trying to run Audacity from the DMG?”
Koz