If you're playing on Windows, here's a way to adjust sound for the game:
- Open the game and hit the Windows key (2 to the right of Space)
- Click on the sound icon on the taskbar (left of the time & date in default) and click on the settings (gear) icon
- Click on Volume Mixer and scroll down until you find the Starmaker app
But yeah, having a way to do this in game would be a lot better.