If enough power and quality speakers you can hear them no problem. For the most part the speakers are shooting the sound over the heads of the passengers so it doesn't blast them out. Just don't stand up.
I'm running 4 pioneer 6x9's with a 2 channel 800 watt amp and the speakers absolutely scream. I ride in the calif. delta and one guy said he could hear my speakers across one of the slews (almost a mile away)!