The Radio is used for "Transmissions" that spawn around the map and gives the player more exploring to do.
To actually get anything from the transmission spots you must play the portal music and put it on the transmission spot.
Transmission Spots (As of 3/11/2022)
Transmission 1, not far from the Radio spawn.
Transmission 2, in the tunnels behind the mirror room.
Transmission 3, does not look like a transmission but it is. After finished the Radio will explode but will not get removed and ignite the closest player.
Transmission 4, this also does not look like a transmission but the entire dark room is the transmission.
Transmission 5, this also also does not look like a transmission but the entire big dark room is the transmission. The Radio will do the same thing as Transmission 3.
Sounds
The 'Portal Music' from the Radio.
The buzz sound effect when you put the radio on a Transmission but the radio isn't playing portal music.