I've heard of people doing that with their cb radios.
Thanks! There's my answer. Why risk my stereo, I'll just pick up a decent specialty speaker.Yes and no with a bigger emphasis on no. The input on your stereo is designed for a very low power signal but your ham radio speaker output is by comparison very high power. It would be extremely easy to overpower the stereo's input circuit. Not to mention the stereo input is a high impedance circuit vs. the ham radio's speaker jack being a low impedance output. In other words the ham audio would probably sound distorted through the stereo system speakers if it didn't damage it's input jack circuitry from overdriving it.
Instead I would plug a small low cost communications speaker into the ham radio speaker jack and mount it closer to your ears so you can hear it better.
As Jerry mentioned a "communications" speaker. They are different than a radio speaker. Their design has a much narrower frequency response. It is a closer match to voice frequencies. Cuts down on the the hiss which is usually the higher frequencies.Thanks! There's my answer. Why risk my stereo, I'll just pick up a decent specialty speaker.