You don't need "OS" to play some music, drive display and talk via USB/BLE. It's trivial task and could be done with a few event loops. A lot of firmwares is being written without OS. May be FreeRTOS/Zephyr to somewhat simplify the programming, but that's definitely not "OS" in a commonly accepted sense. You don't need team of engineers, one hobbyist could easily do that. I wrote firmware for a device of similar complexity (work with ADC, implements USB, BLE, some UI with buttons and leds) and I'm not even a professional.