Can I suspend / hang speakers?

Yeah hanging from pipes mounted to the ceiling Joist would be sonically much better, all things considered(should strongly consider finding best sonic placement "in space first"). simply letting the speakers "hang" wouldn't sound so good, unless the speakers were "so heavy" they wouldn't react much to movement in the speaker.
I would "X" the hanging idea with chains, etc, yes. I mean, if the speaker is haning there anyway(obstructing walkways, etc), you might as well "hard mount" the suckers!

Someone has to say it...

It was Newton's THIRD law of MOTION
vs 1st of thermo.

"For every action, there is an equal and opposite reaction."

If I didn't think Newton was the greatest scientist ever I would have kept my mouth shut. Regardless of the historical slip you make an excellent point regarding the tendency of suspended speakers to shake themselves rather than produce sound.

Thank you Wade- Its been over 30 years since I studied any physics. I was just guessing on which law. I'll gladly take you word for it.

Here is required reading for rigging flyable speakers from ATM-Flyware:

http://www.atmflyware.com/downloads/riggermeister%20guide.pdf

If you don't do it correctly, DON'T do it at all.

Poor Issac must be spinning in his grave! Yes, the voice coil will push with equal force on the driver cone and on the speaker enclosure, but Newton also came up with F = M*A, which tells us that movement of the massive enclosure will be tiny.

The only drawback to hanging speakers by chains is cosmetic. If the wife agrees, go for it.