Hi! If you are referring to the Audio Test, in order to do the test, you must place the doll in the testing chamber room (room with the metal door and a big window). Once the doll is placed, go to the computer, interact with it, hit run tests => Audio => Play Audio Test.
Ah, I see! The burning eye youtube channel has a playlist with all the songs from the game, that I have decided to upload. The audio test music is a song that I have made myself, using the public domain melody of Twinkle Twinkle Little Star, but because I am not certain that it is fully public domain (even though it is from the 18th century), I decided not to upload that one. The song "40", also includes this melody, but it has so many other sounds and instruments playing, I felt comfortable enough to post it.
So, since I am really flattered, that you want to find a little (not even a) song I made outside the game, I will check the rights, and if it is ok, I will post the audio test music today!
Thank you so much, that really means a lot to me <3 And of course, always happy to help!
Play Things - Audio Test here is the music on it's own!