It's much worse than that: if the soundwaves from all the different stereos aren't at least roughly phase-aligned, it's going to sound pretty bad. When working with installed soundsystems engineers typically apply sub-millisecond delays on each channel. Touring setups don't always do this stuff, but they're working with a few dozen speakers from the same manufacturer, not many assorted stereos.

Still, it would be a lot of fun to try. Best to just use FM radio.