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

Actually aluminum is fairly inexpensive in the scheme of things.  It's how it's implemented and tooled that adds any expense.  Vandersteen had it right IMHO years ago when he went 'baffle less'.  It still works in todays market, but many don't love the look of the sock covering.  Teh Treo was basically the 2 in a full cabinet. form.  He was able to make it sound works better though as he had a better price range to work with and is able to use better drivers and components.  

Everything resonates. Surely it's a question of where you put these resonances.

I believe it's far too easy to muffle the midrange with MDF or medite. Many specialist manufacturers purposely avoid the use of MDF for their cabinets, or at the least prefer some form of MDF composites.

Harbeth (along with Spendor?) is the exception, but then their entire cabinet construction techniques are also an exception. 

Aluminum rings like a mutha!!! 

Wood...my Totem Cherry Forest Signatures Sound like a Stradivarius !  Fine musical instruments are made of wood.  Can you imagine a plywood, MDF or aluminum violin 🎻. Yikes!!!

@dave_b - speakers aren't the same.  Musical instruments are designed to resonate.  That's how they get their distinctive sounds.  Speakers, on the other hand, are designed not to resonate.  The less they resonate, the less of their own sound they have and the better they can reproduce the distinctive sounds of musical instruments.