wow, 10 albums, guessing it's one of the following:
1) Download was interrupted and didn't correctly resume.
2) HDD is full during period of download.... Keep in mind that iTunes will download to the local drive if the external drive isn't connected, so check both and the location iTunes reports for the file.
3) Least likely, but perhaps there was a HDD failure.
Review the files size for the songs that won't playback and if they don't look reasonable you can redownload them from iTunes.
Within my library, I have found one song last year that was corrupt and that corruption made it to my backups as well. My situation is probably extremely rare, but was easily addressed by re-ripping the CD. In your case with 10 albums, guessing there was an issue during the time of download, so just use iTunes to re-download again.