All U-M School of Music, Theatre and Dance dissertations can be found in the Music Library. They are located on the 2nd floor, filed alphabetically by the author's last name. All dissertations are cataloged and are searchable in Mirlyn as well. Note that master's theses and DMA papers are not required to be deposited in the Library and in most cases are not available.
If you need to consult a non-UM dissertation, placing an Interlibrary Loan request is often the best option. You can place a request by filling out the form here or by going into the database WorldCat, looking up the item, and clicking on the 'Request via ILL' link.
If the dissertation is more recent, it is also worthwhile to check ProQuest Dissertations & Theses to see if full-text is available there. ProQuest provides full-text for all CIC/Big Ten dissertations filed since 1997; many dissertations from other institutions and years are available in full-text as well.