"Tuned in" is probably the safest option, as LanguidSquid and ssav have mentioned.
"Playing" is applicable, because the word "play" in this context refers to "playback," which has a different meaning than the normal usage of the component word "play." However, usually "playing" is used when referring to songs and music specifically, because "playback" occurs when someone starts a record track, or more commonly nowadays, a digital audio file from the beginning. Usually talk shows or speeches are not pre-recorded, so the words "playback" and its shortened form "play" aren't quite as applicable. However, most people don't regard such nuanced differences in semantics. In usage, "play" would refer to the song or other audio presentation that the channel is broadcasting, not the channel itself.
"Receiving" is correct, although not used very often. It is the most specific/appropriate word to use to describe what exactly the radio is doing, but most people leave it out of colloquial conversation.
However, you could definitely call your radio equipment a "receiver." The transmitter or broadcasting device is somewhere else -- usually where a radio signal tower is located, in the channel's studio.