The mechanics of biological radios are already detailed in response to this question, but I had a more specific use of it that I was planning to use.
My story has a eusocial species that can communicate via radio signal. Their use is similar to pheromone signals in ant colonies, where each hive uses a unique and specific set of signals and patterns to communicate, so a rival hive can’t decode communications easily. This is useful, since queens are hostile and frequently attack each other.
Learning a hive’s “code” gives its rival a massive tactical advantage, which encourages the species to quickly learn to do so, as well as making radio communication much more complex to avoid decryption (inadvertently driving a spike in social and logical intelligence).
As this species develops, the queens of different hives realize the advantages of temporary alliances and treaties with other hives. However, complex communication with another queen requires learning their code, which no queen will allow, since it’s a death sentence. Creating a decrypted, universal signal between all queens is considered, but it allows anyone in range of the signal to “listen in” on deals best kept discreet.
Eventually, a few geniuses develop a simple form of sound-based verbal communication that allows queens to have limited negotiations. The language spreads and eventually becomes universal, as alliances quickly become essential to a hive’s survival.
While it never replaces radio-based communication, the language does gain uses within the hive as well, since sound is more difficult to intercept and track than radio signals.
Is this a realistic development for a eusocial species? The only alternative I can think of is visual communication (like signing or writing) but they seem limited enough to justify sound-based speech (at least at first).