To stain or not to stain..... hmmm.....personal choice. I have never used any stain on my boxes, but that is only 'cause I want what God made to show its own beauty, and it does so quite well without my help, although I would'nt tell someone mine is better because it isn't stained.
I do not believe that stain or finish,( properly applied) has any effect on sound quality of a call, though I would never enter a box call with stain in any competition and expect a favorable outcome from a judge.
Sound does not come from the wood used, rather tone comes from the wood used, and you can achieve different tones(sometimes folks say,it has a different sound when what is heard is a tone or note) by using different woods in your box calls, scratch boxes, etc. For example most folks are familiar with the popularity of a Purple Heart Paddle on say a Mahogany box, and the ability of that combination to make a reliable and consistent yelp, but, you'll have a different tone if you change the wood your striker is made from, and or the box. Different woods = different tones due to density of the wood.
The tone can also be manipulated by changing the thickness of the sound boards, over which the striker glides. I measure my sound boards by the thousandth of an inch, knowing that by doing so I can change the tone and much more so, the pitch of the call. Also the size of the sound chamber plays a large role in a calls ability to project sound or noise. For example a long box is generaly louder than a short box. All box calls can be aimed and made to be directional, by pointing the chamber towards the target. Musical instuments are a good example.
All sound is made from vibrations, which generate soundwaves, with varying distances betwwen the waves and different durations of the wave. Sound from a box call , comes from the vibrations made when the paddle glides over the sound boards of the box. The sound boards vibrate and too will the paddle if it isn't being held too tightly. Apply pressure to either the boards or the paddle and you can change the pitch and tone.
Vibrations will move through stain unimpeded and unless too much finish coat is used,(ie oil, laquer, poly, etc) it will also travel well through finishes.
I used to think that the glue used in the glue up boxes impeded sound quality, but I have heard some darn nice sounding glue-ups, so I no longer make that statement, and anyways it isn't used on the sound boards.
Well, thats my nickels worth, and I sure don't have any intention of telling others they are wrong, just trying to be informative while sharing years worth of experience. Take what ya can use and leave the rest.
GOOD LUCK, GOD BLESS, AND STAY SAFE !
HOOSIER HUNTER CUSTOM CALLS
MOSSY OAK PROSTAFF-TURKEYS