We've seen an increasing number of reports from people who are finding customs in the database with broken links. Unfortunately this is not something we have control over as the problem is due to an invalid or expired link from the file hosting site. We cannot upload songs we did not create to a file hosting site we do not own for a user that we cannot access. If you want the link to be fixed please contact the user who originally submitted the custom. If the user is no longer active you are welcome to ask around if anyone else has the song or even chart it yourself!
Thank you for understanding!
On a similar note, please don't ask for us to "fix" the links to songs that are official Rock Band content. We assure you those links are definitely not broken and should redirect you to a place where you can purchase the songs. Thanks!