To be able to decode data the application needs access to the microphone.
Having access to the microphone is considered to be a sensitive permission and you have the right to know what the app does with that access:
The data recorded from the microphone is only used as input to the decoder, meters, and functions such as spectrum analysis for visualization of its frequency content.
The application may use a very small temporary buffer in volatile memory and it will constantly overwrite this buffer with new data from the microphone.
The resulting output from the decoder is the only data that gets stored in persistent storage on your device.