This is the traditional bugle call known as Assembly, which is used by military forces to command the troops to gather for inspection. A similar call used by some forces in some countries is called Fall In. See the links at the left to download the sheet music or audio files. More bugle calls are listed in our article about the history and cultural impact of Bugle Calls
The image below is the first page of the sheet music, and here you can download the full sheet music for Assembly (bugle call) in PDF format.