Updated To Mavericks, Now Audio Quality Blows.

Try changing all the song's Equalizers to anything, then changing them back. I generally have EQ turned off on all my music, so I selected everything went to Get Info (Command + I), Options tab. Change the EQ/Equalizers to anything, then apply those settings (press return/enter). It may take a while to process and change everything. Once it's done, do the same thing and turn the EQ off (or back to whatever you want it at). Press return/enter and things should be back to normal when it finishes processing all the songs.

EDIT:

Well, that workaround didn't work for very long.

Did a little more messing around. I would suggest simply reinstalling iTunes. It's very easy and doesn't make you lose any songs. It took 5 seconds to do after it was downloaded and it fixed all my sound issues that you're describing. Here's a support document with details. http://support.apple.com/kb/HT2311

Summary:

-Download the iTunes dmg from http://www.apple.com/itunes/download/

-Once it's finished, quit iTunes if it's running.

-Open the dmg file you downloaded.

-Run the installer.

Once it's done, run iTunes and everything will be back to normal. Quick and easy.

Is this only affecting Mav? I'm still on iTunes 11.0.5 and SL. Just wondering if this is a Mavericks only issue

I'm still on SL and had the same issue, had to go back to 11.1.1 thanks to my Time Machine!

Very annoying...

Deleting the .plist files like i mentioned before is the way to fix this issue. .plist files are like a configuration file, sometimes they get corrupted though. Deleting a .plist file will not delete your data or change you settings. All you need to do is delete the two .plist files and you will not have the problem again.

Just delete these two files and restart. Your iTunes will be back to normal after that.

I don't seem to have those two files, are they hidden or something?

Can you see your Library folder?

Yes I can see that, and the Preferences folder, but the two files you mention aren't in it. There are lots of other .plist files, the only iTunes one is com.apple.iTunes.secure.plist

Just reinstall iTunes then. I just made a post about it yesterday: https://discussions.apple.com/thread/5469255?answerId=23640130022#23640130022

That means you're in the wrong Library folder. Keep in mind your Mac has three different Library folders: Yours, Public, and System. You will find the files in your personal (user's) library folder.

While pressing the Option key, go to Go > Library. That modifier should make finder show the folder in the Go dropdown.

You can also manually go by writing down /Users/your_user_name/Library

Reinstalling will fix it too, but I guess it's shorter to erase and restart.

Cheers,

JJR

An update to iTunes has arrived that fixes this problem. It worked for me. 🙂

This worked great for me. Try Audirvana aswell perks up iTunes big time.

Tried deleting all iTunes plist files - didn't fix the problem for me. 😟

I did both aforementioned solutions. First, I redownloaded iTunes from apple.com. Once iTunes reinstalled itself, I opened it up and tried playing music. Sound was still bad, so I deleted the .plist files and restarted. Those two things together fixed the problem for me.

Cheers