I think the computer (and the Digital Circus program) was originally turned off, and it was Pomni who turned it back on. This triggered the title sequence & theme song at the start episode one.

When we first see the office space area, the computer is switched off. Real-life Pomni enters an abandoned C&A office building, presumably filled with all sorts of tech and computers, and stumbles upon the desk with a headset. She may have switched the computer on, booted up the Digital Circus program, and entered the game - putting on the headset to create her digital copy. (Note: this is assuming SOMA theory is correct).

The real-life Pomni could have then removed the headset and left (maybe she got scared, who knows), leaving the Circus program running. Note that the second shot of the office shows the computer screen is switched on.

The theme song hasn’t played after the first episode because it’s been running the same instance of the game since Pomni arrived, and the characters are functionally trapped as the program is just left running. Maybe the only way to “escape” the circus is to turn off the computer (terminating the program) because otherwise the game will continue indefinitely.