Hi have you tried the apple website? click here (Document: "iTunes and QuickTime for Windows: Songs and other audio don't play correctly").
Or try these that I cut & pasted from the Help section I found in my iTunes software (soryy it's o long... but some/all of it may be useful to you & anyone else browsing this thread). P.S. This was copied from iTunes Version 6
"I can't play a song"
***If you can't play a song, check the following:***
- If you can't play a song that you purchased from the iTunes Music Store, you may have exceeded the number of computers on which you can listen to your music purchases. Once you've "authorized" five computers (by playing songs you've purchased on those computers), you can't listen to any purchased songs on a sixth computer until you deauthorize one of the already authorized computers. To deauthorize a computer, choose Advanced > Deauthorize Computer (the computer needs to be connected to the Internet). Then choose "Deauthorize Computer for Music Store Account."
- If you're listening to a shared library or playlist, iTunes skips any purchased music in the list if this computer hasn't been authorized to play that music. To listen to a purchased song in a shared library or playlist, you need to double-click the song. If your computer is not authorized to play songs purchased by the person who is sharing the song, you'll need to enter that person's Apple or AOL account ID and password to hear the song.
- You might be trying to play an AAC file that was not created using iTunes or downloaded from the iTunes Music Store. Songs you download from the iTunes Music Store or import into your library using the AAC encoder are encoded using the new MPEG-4 AAC format, and will play in iTunes and on your iPod. Older AAC files that you find on the Internet or elsewhere won't play in iTunes.
- If you're trying to play a song on a CD or DVD that you burned using iTunes, make sure the equipment you're trying to play the CD or DVD on is appropriate for the disc's format. For example, if you burned a CD-RW disc, you can only play it in your computer and some newer CD players. If you burned an MP3 disc, it should work in your computer and on special MP3 CD players, but not on standard consumer audio CD players.
- If you have a slow Internet connection (128 Kbps or less) and are trying to play a song preview in the iTunes Music Store, the preview may not play smoothly. Choose Edit > Preferences, then click the Store tab at the top of the window. Click the "Load complete preview before playing" checkbox. Then try previewing the song again.
- Use Windows Update to make sure you have the latest Service Pack. Also check your computer or audio card manufacturer's website to update your audio card drivers.
- If you're having trouble playing music you've purchased from the iTunes Music Store, make sure you have the latest version of QuickTime.
- If you're listening to a playlist that includes a song preview, the playlist stops after the song preview. Double-click the next song in the playlist to resume playing the remainder of the items in the playlist.
- If the song doesn't sound right when you play it, you may need to change your QuickTime settings. Choose Start > Control Panel. Then double-click QuickTime and click the Audio tab. Click "Select Audio Playback and Recording Devices" and choose a different device. If the song still doesn't sound right, try turning off the iTunes Sound Enhancer in the Playback pane of iTunes Preferences. If that doesn't work, see this article on the Apple Support website: click here.