You cannot hear any sound when you play iTunes but can hear sound from your speakers through normal activity, (ie: beeps, install / uninstall sounds, etc).
The default sound device may need to be changed. Right click on the speaker, (lower right corner of the screen), and open the volumn control mixer. Play a tune in iTunes to see which device is playing, (look for the up and down bar movement). Most likely the one playing the song is not your speakers but another device. iTunes thinks that device is the default sound device.
Change the default sound device to the correct one and the issue is resolved.
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