My apologies for being repetitive. But the information below may help members that are new to this discussion regarding severely distorted playback of mp3s and media not purchased from the iTunes Store. There are quite a few Apple Support discussion threads on this topic. Try this search for "distorted" with activity in the last week and you will see many active discussions. shin_shin has identified the lack of SSE2/SSE3 on older Pentium 3 and AMD processors as the possible cause for this issue in this thread.
Some members chose to stay with iTunes 10.5 and wait for a patch. If you wish to go back, here are Apple's instructions on unistalling iTunes 10.5 and other components, and the link to iTunes 10.3. (iTunes 10.4 could not be located on Apple Support).
iRemoving and Reinstalling iTunes, QuickTime, and other software components for Windows XP
http://support.apple.com/kb/HT1925
Removing and reinstalling iTunes, QuickTime, and other software components for Windows Vista or Windows 7
http://support.apple.com/kb/HT1923
Tunes 10.3.1
http://support.apple.com/kb/DL1103
If you don't like following links, the following instructions may also be helpful, too (Warning - All libraries and playlists may need to be rebuilt using this method). You will either need to rebuild your iTunes libraries or you may be able to find the required "iTunes Library.itl" file the following folder: \My Documents\My Music\iTunes\Previous iTunes Libraries. Restore the previous file to \My Documents\My Music\iTunes (copy and rename it to remove the date info).
Here are revised steps to downgrade from iTunes 10.5 to 10.3, 32 bit
- Copy My Music/iTunes folder to a safe location
- Control Panel - Add/Remove Programs - Uninstall iTunes
- Control Panel - Add/Remove Programs - Uninstall Bonjour
- Control Panel - Add/Remove Programs - Uninstall Apple Application Support (optional)
- Control Panel - Add/Remove Programs - Uninstall QuickTime (optional - QuickTime will be automatically reinstalled with iTunes 10.3)
- Run CCleaner Registry Cleaner and Restart (optional/recommended - dll error during start-up will be resolved after iTunes 10.3 is reinstalled)
- Download iTunes 10.3 from Apple (http://support.apple.com/kb/DL1103)
- Run iTunes installer
- Import music file containing mp3s (iTunes - File - Add File to library)
- Test playback of files (should be good).
- Restore the files you saved in Step 1 by copying the folders from back-up file to the following folders in My Music\iTunes\iTunes Media:
- - Automatically Add to iTunes
- - Books
- - Downloads
- - Mobile Applications
- - Music
- - Podcasts
- Restart iTunes and you should be back in business.
If your iPhone is iOS 5, you will need to refrain from syncing with iTunes until Apple sorts this problem out. Interestingly, I also needed to unistall and downgrade Safari 5.1.1 back to 5.0.5. Safari 5.1.1 broke QuickTime and files played back with video, but no audio, even after iTunes was successfully downgraded.