Singer/songwriter Tom Waits is recording the follow-up to his critically acclaimed 1999 album Mule Variations, according to a spokesperson for the raspy-voiced singer. The still-untitled LP is not yet scheduled for release. Waits also plans to fly to Copenhagen with his wife and collaborator, Kathleen Brennan, to work on a contemporary avant-garde theater version of the opera "Woyzeck," with theatrical director Robert Wilson. Waits and Brennan previously teamed with Wilson for the theater pieces "Black Rider" and "Alice," the latter an opera based on Lewis Carroll's famed children's story "Alice's Adventures in Wonderland." The singer also is producing the upcoming album by blues singer John Hammond. Waits wrote the song "No One Can Forgive Me But My Baby" for Hammond's Got Love If You Want It (1992).