The original letter (according to this article on CNET) was received last Thursday and demanded that the institution take remove the site and delete the offending files.
Puzzled by this, administrators searched the site for files ending in all of the popular media extensions and only found one .mp3 file on the entire site. It turns out, that file was an a capella song performed by astronomers about quasars.
It appears that the instigating professor was named Usher and the RIAA's bots were triggering on the combination of Usher and .mp3 to guess that the song was a pirated song from the artist Usher.
As usual, the RIAA claims that they made an "honest mistake" and that their policies don't require anyone to actually listen to the music.
Unfortunately, it appears like we may all be in for more of this, since the DMCA doesn't require a higher standard and only provides legal liability for the complainant in the event that they "knowingly and materially misrepresented" the information used to send the take-down notice.
As usual, more information on the DMCA can be seen on the EFF web site.