Room Treatments: Paneling and Varnish

I have greatly improved the sound of my system by covering the wall behind my speakers with birch plywood panels butted together. The paneling has to be finish-coated for long term installation. This leads to a question of varnish formulas and their acoustic effect.

I'm inclined to avoid water-based, acrylic varnishes since that would effectively put a layer of plastic over the wood. Would a typical oil-based varnish like Minwax be a better bet acoustically? All suggestions welcome. Thanks.
I would jsut use a minwax type stain on it a nutral color or whaetever you perfer this would add moisture to the wood and prevent stuff from soaking in that would normally stain with out going to a varnish coating. If you have to do a varnish stain it first and you can try a tung oil, it is basaclly oil base varnish or polyurithan mixed with thinner so it puts on a very light coating just enought to stop stuff from soaking in.
You may be thinking more about the difference between a surface finish like polyurethane or oil varnish versus a penetrating oil finish, rather than synthetic vs. natural. Varnish is a lot like paint (but clear, of course) in that it forms a protective layer that sits on top of the wood. Penetrating oils (like the typical Minwax finishes; VelvitOil is terrific, if you can find it at a wooworking store) carry resins into the pores of the wood and harden the tissues of the surface layer of wood, rather than forming a coat over the wood. So, wood finished with a penetrating oil finish leaves more texture to the wood.

Both varnishes and penetrating oil finishes can come in either natural or synthetic forms. For example, Tung Oil is a natural penetrating oil finish, and Minwax and VelvitOil are penetrating oil finishes made with synthetic resins.

All of these differences in finishes may have an effect on reflectivity, absorption, diffusion, and the flexibility of the surface they are applied to. I would think you could hear these differences if they were big enough, but I can't give you any empirical advice.
By the way, the labeling on wood finish products is horribly non-standard, so it can be tough to tell what kind of finish you are looking at in a store. Generally, a surface-type finish, like varnish, is applied like paint. Good brush technique makes a difference. A penetrating finish is wiped into the wood, and technique is not so important. In these modern times, there are crossover products, like wiping or gel polyurethane varnish, which takes a synthetic resin product that is associated with the toughest surface finish (famous for its plasticky look when built up many layers thick) and makes it behave like a penetrating finish.