Which material sounds better for speakers construction? Wood, Ply or MDF?

Which ever material is lower mass would be better. High mass materials have a low resonance point. Very difficult to deal with low frequencies. 

Aluminum has never seemed viable as a musical speaker cabinet material. I don't think it will stand the test of time. 

Plywood is by its nature a composite material that has self dampening properties. When used properly this can sound excellent. I think some old violins are made with plywood. Not sure. 

WATT uses inert cabinets.  Yet,*internal bracing* with all the wood combinations mentioned, will drastically cut down on resonance and vibration. 

Keep in mind, drum sets consist of wood plies or solid wood, and they get their unique tone from the choice of the various wood used.  Different wood types resonate at different points.   MDF would make for a very dead sounding drum because it resists resonating.  Who knows?   I would think the drivers and the quality of the crossover would make the big difference.

If we read with an open mind - "Why wood sounds good":

http://www.mother-of-tone.com/mother.htm

Very interesting indeed.

^By that argument, speaker cones should be made of doped skin, like an Ed Gein lampshade.

Speakers are not musical instruments. Their purpose is to create a perfect replication of the recording through air movement, with minimal influence on the signal from the cabinet or diaphragm materials. Anything that resonates during this process is a coloration, an alteration of the original sound. If you use a spruce horn to project the sound of a recorded violin, essentially what you’ll get is the sound of a violin that’s been altered by further wood resonance. One might as well encapsule the violin within another violin. It will no longer sound like a Stradivari, but will sound like a violin made of that cheap, new-growth spruce that the builder got from their local lumber yard. This still might be pleasing to the ear, but if it resonates more (or at a higher frequency where our ears are more sensitive) vs the plastic horn, it is a greater alteration of the recording.

This is why speaker manufacturers tend to use self-damping materials: plastic/ paper/ceramic cones, fabric tweeters and MDF cabinets. Their resonance tends to be of lower amplitude for a given SPL and therefore, they have less of an impact on the original signal. The perfect speaker would be one that produces sound waves while imparting absolutely zero resonance of its own. A solid wood enclosure won’t get anywhere close to that ideal, because it’s highly resonant by nature - great for instruments, not for speakers.

How can you ask that question