Im Radio heißt es, der Bus komme wegen einer Umleitung etwas später.

Im is a contraction of in dem.

So im Radio literally looks like in the radio, but idiomatically it means on the radio or in a radio broadcast.

So this phrase is not about something physically being inside a radio. It means the announcement was heard through radio news or a radio program.

Es heißt is a common impersonal expression meaning something like:

• it says
• they say
• the report says
• it is said

In this sentence, heißt es introduces reported information. It sounds especially natural in news, announcements, and formal reporting.

The es here does not refer to a specific thing. It is just a dummy subject, similar to how English sometimes uses it.