I am getting a radio soon and would much rather have the sound using the speakers on my iMac than its own. The radio has a speaker jack that claims it outputs more than 2.4w of power. I am not too familiar with how much power these audio lines normally use so I was wondering if I can plug the radio right into my input jack or if I need to buy a special piece that allows me to do this safely?
In general, if you're going to feed a speaker output into a microphone input, you should buy a "line-in adapter" that attenuates the signal. This protects the input and gives you better quality sound.
In practice feeding the signal in directly "probably" won't damage the microphone input, so long as you don't crank the volume all the way up. 2.4w is not that much.