Acoustic with wood panel

It won't be any different than a bare wall behind the speaker. Any effect would be at very high frequencies and would not be discernable.  

Your description isn't very clear. However, if this product is bonded to the drywall, it should reduce vibrations and structural resonance. I suspect, you may find a little added liveliness to the sound compared to drywall. 

Well I'm not sure where exactly you want to put MDF and what you have right now.  If you have drywall and putting an MDF panel in front of it it will be roughly equivalent.  Depending on the location and your room's existing layout and treatments that may not be what you want there.

The main reason is decorative. However, I want to make sure that this will not affect the sound. I plan to put diffusers and bass trap if necessary, I'm afraid that because of the hollows it will affect the sound

It sounds like you want to put a flat piece of MDF behind the speakers?  They'd sound about the same as bare drywall.