All credit for this goes to user aditya_t90 @ xda-developers
I wanted to post this here because I read his/her guide but when I did it on my phone I couldn't find any .odex files at all. I assume this is because I'm using a de-odexed ROM. I decided to take a chance and go through with the operation anyway and it worked.
Do it like so:
You need a rooted phone to do this!
Go here: http://androidhd.blogspot.com/2010/0...one-style.html
Scroll down a bit and download the music app 2.1 apk to your computer.
Grab your phone. Use a file manager like Root Explorer (or do the next steps with the command prompt.)
Navigate to system/app/
Move MusicPlayer.odex (I couldn't find this. It doesn't seem to matter as it worked fine for me anyway) and MusicPlayer.apk to your sdcard under a folder named for example backup.
Next mount the phone to your PC in mass storage mode with your USB cable. Transfer the MusicInstallable.apk to the phone.
Pull down the notification bar again and turn off the drive. Disconnect USB cable.
Using a file manager, find the MusicInstallable.apk that you just transferred and click it to install.
This worked for me.
I'm not responsible for anything if this messes up your phone, computer or relationship. No anchovies unless otherwise specified.