OK, I'll bite!
A teenager I know wanted a "solar powered PA system" you could say. Those are the most "eco friendly" of sound sysems since the first priority of the is to concerve very limited (and variable) power while providing the maximum SPL attainable. This means high, ultra-high efficiency across all componens of the system. Sure, you have Class D...of course but, the speaker efficiency is far, far, far more important so the pathetic consumer speakers were eliminated immediately. KEFs and Elacs demand 10 to 40 times the power to run so not going to happen.
He realized quickly that he would have to build the speakers he required, that was fine as he had opposing thumbs, built car audio systems without hurting himself and owned basic power tools Looked at several designs, measured out his specific ergonomics (place handles and box size to fit his frame) went with pro drivers with a pair of 2.5 way mains (101dB 1w/1m) that weighed around 25 kilo or so and fit his shoulder width no probem. Sure, they handled a ton of power but meant to be driven at 25 to around 200 watts max--efficiency was #1.
Went with 15" subs, four of them to boost efficiency and narrow enough for one person to carry. Very tall to allow a larger air space but narrow as possible and fairly shallow and around 30 kilo... Neo magnets to cut weight but not inexpensive by any means. He did the research on stout car amps that were the most efficient and with a 4 to 8 ohm load, would laugh at such a high impedance as they were rated to 1 or 2 ohms.
His solar pack was several KW/h, MPPT controllers, LiFePO4 packs at 12.8V nominal and when assisted by solar cells in the summer sun, he could go until far into the night at arouund 2 AM. Hey kept an eye on the current output display and that was his ruler to get it done correctly.
Was his system "eco-friendly"? Looking at 101dB one watt/one meter and class D amps using a battery power supply (no inverter losses) he did a very good job. Now multiply how many hours of use and how much power is needed with 101dB VS 85 to 87dB speakers. He is very happy with the speakers and uses them for hi-fi speakers during the week and uses the solar system during the spring/summer months around twice a month. Look at the cost of shipping large speakers half-way around the world as big boxes in even larger cardboard boxes with foam to protect them. He purchased around 5 or 6 sheets of plywood with no shipping materials and built the things. Sure, the drives had to be shipped around 300 miles but the amount of materials is far lower.
An older friend of mine wanted me to refurb some fully horn loaded speaker he bought used when he was a teenager (late 60's) Tore the horns completely down, resealed the wood, measured and built a much better and accurate crossover network and made them sound better. Those things were 103dB 1w/1m being fully horn loaded and I ran them on a 10 watt chip amp for entertainment...geez, those things were LOUD! Not the ultimate in sound quality, far from it but he liked them and still uses them to this day. Designed a subwoofer for him, used an 18" PA driver with a small Class D PA amp as it attempted to keep up. This guy is happy and has been using the same speakers for over 50 years and they still work (crossover needed to be changed though) Fully horn loaded meant the drivers were never exposed to UV from the sub and being so incredibly efficient, for home use he was fine with 25 to 75 watt per channel amps. Those speakers are in use at least 30 hours per week, now part of his HT system so multiply 30 X 52 weeks X 52 years.... He bought his second pair of speakers about two years ago, a pair of studio monitors for his other room that he hangs out in. The garage provides tunes via a silver boom box...
So if you want a lower carbon footprint, go with high efficiency speakers. Class AB amps are pigs, their efficiency is poor at less than 20% at lower power levels with Class D at 70 to 90% or higher at the same lower power levels. If you use air conditioning, lower heat produced will demand less power to cool down Class D. Even if you can't use very high efficiency speakers, be aware that a 91dB 1w/1m speaker uses 4 times less power than a 85dB 1w/1m speaker. The speakers I use are "large bookshelf" speakers (my wife would claim otherwise) but at 98dB 1w/1m they require very little power to get me into trouble.
The last thing is to avoid disposable products--a lot of audio gear won't have parts available to fix them (if you can fix them) in 10 years. Get devices that tend to run cooler to extend the life of the device, try to find products that are known for their reliability and don't beat them into the ground. I've had my very efficient speakers built up for 5 years now--I am content with their performance and they will be keepers.
Still looking for a programmable chip amp board, a 2.1 with DSP to make a "boom box" that runs on 18VDC 5Ah power tool batteries. Yeah, very limited in output power at around 20 watts into the 8/4 ohm speaker loads and maybe 40 wats for the 4 ohm sub but great as far as a carbon footprint goes!
For the most part, audio is a grossly inefficient beast that very low efficiency speakers, horrid Class A and tube amps are celebrated while everyone "needs" kilowatts of power even if they don't. After all, the "best" stuff burns more power at idle than all the LED lights in your house...the good stuff! You can get Class D amps, low power processors and high efficiency speakers but they tend to be more fringe products--that is changing slowly as the world changes. Think of the carbon footprint of record players! Ack!
Good luck in your quest, a ton of factors come in play but entertaining to get it all working the way you desire.