Music works on iPhone but not on iTunes

Question

A really good chunk of my music doesn't work on iTunes anymore - I'll try to play it but get the message that iTunes can't find the file. It definitely still works on my iPhone, but I'm afraid to sync because I'm worried that what music does work on the iPhone will stop working when it syncs with iTunes.
What can I do to get the music that works on my phone back onto iTunes? I probably haven't synced the two since maybe the holiday season; I don't remember how long this has been happening, but it's not new.
Thank you in advance!

Answer 1

I figured out how to remedy this last night. I had to pull my music off of my iPhone since it was no longer on my hard drive -- I used a free program called [Senuti|http://www.fadingred.org/senuti> to transfer the music (note: you have to download the Beta, which is linked at the bottom of the page, for it to work with an iPhone).
The program's website has instructions on how to do the transfer without your phone syncing to your computer (which might delete the music from your phone that's missing in iTunes).
Senuti works by figuring out which music is already on your computer, and copying any music that isn't already there. The problem with this is that all of your music is listed in iTunes even though the files aren't there, so Senuti won't copy anything without some extra steps first.
I had to delete all the songs in my iTunes library (from within iTunes -- and make sure you choose to *keep the files* on your hard drive).
Then in iTunes, I chose *File >> Add To Library...* (or Command + O). By default my iTunes Music folder was already chosen, but you might have to navigate to it and then click "Open."
This will add the files that you do have back into your library. After this, use Senuti and it will copy over all of the missing files from your iPhone to your computer.