Test sound levels at each bike (including the instructor's), not at the speakers. Do your normal cueing while you're testing it, so you get your voice level in there, too.

This is tough because a lot of instructors and students like their music VERY loud. I actually complained about it at a fitness convention I went to this fall, and there's one aerobics instructor at our gym that if I'm going to take her class, I wear earplugs (bright orange ones so everyone knows). But when I measured the levels in her class, it was "only" 86-87 dB - truly painful for me, but well below OSHA regulation for a one-hour class.