From the NY Post:

Beyonce Knowles' flawless performance of “The Star Spangled Banner” at the inauguration was entirely lip-synched, Page Six has confirmed. A rep for the United States Marine Band revealed that she decided to perform to a pre-recorded track at the last minute.

“All music is pre-recorded for the ceremony because there are so many eventualities and conditions that day,” Kristin DuBois, a rep for the band tells Page Six. “We performed, live, the band. But we received last-minute word that Beyonce was going to use the pre-recorded vocal track. Those were the instructions we were given. We don’t know what the (sic) reason why.”