Among the icons you might see in your iTunes library after enabling iTunes Match is this unfriendly guy to the left: a cloud with an exclamation point in it. No, it's not hinting at a chance of inclement weather; it means that iTunes Match has encountered a problem while scanning the track in question.
Of course, iTunes isn't very helpful about it: Enable the iCloud Status column in iTunes and you'll simply see "Error"; click on the exclamation-point-bearing cloud icon and you'll get this "helpful" dialog box.
Generally, what this really means is that there's a corruption somewhere in the file - it may be one that you can't even detect by examining the file or even by listening to it.
But if you have found yourself confronted with songs that iTunes Match simply refuses to associate with, there's a relatively simple solution:
First, convert the track in question to an AIFF file. (AIFF is a raw, uncompressed audio format.) To do so, go to iTunes's preferences, click on the Import Settings button in the General pane, and select AIFF Encoder from the Import Using drop-down menu.
Select the track that you're having problems with, control-click on it, and choose Create AIFF Version. iTunes will make another copy of the track, this one encoded as an AIFF - to differentiate between it and the original track, make sure you've enabled the Kind column by control-clicking on the column headers and selecting it. (You can also convert multiple tracks by command-clicking them to select them, and then choosing Create AIFF Version.)
Once you have a new version, delete the existing version from your iTunes library by selecting it in the Music section of iTunes and hitting the Delete key (or by control-clicking it and choosing Delete from the contextual menu). You'll be asked if you really want to delete this song from your computer. Here's iTunes Match's dirty little secret: Despite the error, your iCloud library does contain an entry for this song, so make sure you check off the box that says "Also delete this song from iCloud." (If you don't want to delete the original file, make sure to click Keep File in the subsequent dialog box; that way it'll stay on your computer, in case you need to refer to it again.)
After you've removed the error-laden track from iTunes, control-click on your new AIFF version and choose Add to iCloud. iTunes Match ought to do its thing and match or upload your new version to the cloud, sans errors.
Now, one downside to AIFF is that it's a lot larger than compressed audio, such as the AAC format that Apple uses. But, if your track is matched to iTunes's database (not just uploaded), you can delete the AIFF version in your iTunes library - this time making sure to not delete the version on iCloud as well - and re-download a pristine 256-kbps AAC file. (If you have a lot of files you need to fix, then you might take an extra step and use iTunes to convert them back to AAC files before uploading, to speed up the process.)
You can skip a lot of the above conversion hooplah if you use a program like Rogue Amoeba's Fission audio editor. Because Fission is lossless, you can open an AAC file and re-save it without any degradation in quality. Depending on the errors in the original file, the new version that you save might upload to iCloud successfully. (It did in at least one of my tests, even though Fission noted that there were errors in the file when it re-saved it.)
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