Bluetooth sound streaming with arduino
What you are describing will not realistically be practical with a classic AVR-based Arduino.  You would need at least one of the ARM or ESP8266 based varieties, or a DSP architecture, or even one of the cheap orders of magnitude more powerful SoC systems.

However, to answer your literal question, the phone would know that the data is an audio signal, because to get the phone's stock software to treat the signal as audio, you would have to be operating the your creation's Bluetooth system in an audio specific profile.  In the case of the RN52, the firmware of the module restricts you to choosing between these modes; without changing that firmware, you can't for example use that module for some other type of Bluetooth operation like being a keyboard or a serial device.