I wanted to know if the same 3.5mm cable that transmits the sound from the speaker to the player can be used to transmit the power required to drive the speakers.
Are they in commercial use?
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70-volt floating line [also known as Constant Voltage systems]
The idea behind 70 Volt and perhaps more common in the UK, 100 Volt line systems is that you have an amplifier that puts out 70 Volts at maximum output which for argument's sake is say 120 Watts RMS.
You can now attach speakers via individual transformers which will take some power from this line. Each speaker may be set to say 10 Watts so you simply add up all the speaker's power settings and as long as it is not more than 120 Watts you are all set.
Typical speakers (or the transformers designed for this work) are rated in Watts RMS and usually have many 'tappings' to allow you to have greater or lesser volume from any given unit. A typical 'naked' 70 Volt line transformer would have labelled secondaries for 4/8/16 Ohm speaker (allowing you choice of actual speaker) and you would select the primary tapping to suit how loud you want.
This is all relatively easy so that at install time you simply add the individual speaker 'wattages' together and ensure your power amp will give at least this much. Basically the 'hard' maths is done for you.
You can get power amps with transformers already fitted internally (your safest bet) or you can add an external 'step up' transformer to take say a 120 Watt power amp rated for 8 Ohms speaker, up to 70 Volts. Note that not all power amps are really happy driving a transformer and you should enquire about it's suitability if you go this route.
See http://www.jblpro.com/pub/manuals/pssdm_2.pdf for mind-numbing technical detail ;-)
In almost any professional setup beyond very small setups, the speakers are passive. An amplifier (power amp) powers the signal and sends the signal down the line that drives the speakers. Multiple speakers can be driven from a single amp, but the power is divided between the speakers and they all play the same thing (since they are being driven from the same signal.) There can sometimes be many, many speakers driven off the same amp, such as in the case of a line array (which increases quality by spreading the sound production over many drivers, but with the same amount of overall power.)
You don't typically see this done over 3.5mm connectors, but it is regularly done over the big brother of 3.5mm, the 1/8 inch connector. It is also sometimes done over locking SpeakON connectors to prevent accidental unplugging of the cable while the system is in use. This is also the same approach used for any receiver based home theater where the use of banana plugs or bare wire connectors is common. There isn't any reason you couldn't do it over 3.5mm connectors as well, but I think it is generally avoid to prevent confusion amongst consumers. (Connecting, say, a pair of headphones to a powered output would likely instantly blow the speakers in the headphones.)
Active speakers (the kind where you send a line level signal and plug them in to their own external power) are really a more recent (relatively) concept where the amps got small enough and cheap enough that they started placing amps directly in speakers to make it easier to setup and more portable. Traditionally, such systems have been more common in basic setups though as they have some drawbacks, such as being more expensive to repair and upgrade, more limited in their adaptability and often being more expensive for their quality. Recently though, they've been getting more higher end professional usage where space, portability or less cable to run to the speakers is critical.