If you have access to the same digital stream for subscriber No. 1 and subscriber No. 2 (sender and recipient of messages), then you can select some sequence of bytes from the stream to use as a key or initial data to create a key.
If the digital stream is not random (for example, it is an Internet radio stream), but you select data from it randomly, then ... it can also be a random set of "low quality" numbers. For example, your algorithm will choose numbers that will be the same several times in a row, because. at these moments, the signal level (the volume of the Internet radio stream) changed little.
How to improve such a set of numbers? The simplest options: discard repetitions, skip (discard) close values. This is easy to do in the same way for the sender and receiver of messages and generate the same keys. Additionally, known techniques may be used. For example, capture a fragment of an Internet radio stream and archive it as a ZIP. Then select a portion of the bytes from the ZIP file and use them with the bytes from the digital stream to generate an encryption key.