That's fine, plugging in a line-level output into a mic input; you're not overpowering the input. However, the problem lies in plugging a high-voltage output (e.g. the signal from your speakers) into a mic input (which is designed for mics - completely unamplified input).

Do a search for "input" or something along those lines in this section of the forum, and you'll find some useful info from Doc Simon. I think he's away for a while, but I'm sure he'll jump in on this thread as soon as he gets back. In the meantime, make good use of the knowledge that has already been dispensed in this section and "search".