My great grandfather disappeared in 1930 and the only record I have of him is these photos. He fathered my grandmother in 1914 in Australia and had a second family in the late 1920's. I have only just discovered his second family via ancestry DNA testing. His grand daughter provided me with the photos. I can tell that he's a musician but I don't know what branch of the services he was in. I can only assume that it's a US uniform as it appears to resemble a First World War uniform.
There's not much that can be said that hasn't already.
That's the uniform of a enlisted United States Army "doughboy". The uniform dates from between the first and second world wars.
The insignia is that of the US Army Band.
Note: The above insignia is "band leader". I assume the star above is the reserved for band leader.
That's as much as I can get without more details. I recommend you make a request to the US National Archives, you should be able to get details about your great grandfather.