Dear colleage,
I say colleage because I also work in a Natural History Museum.
You can use the .ini file to set all the boards the same output volume. Example and explanations are provided on https://robertsonics.com/mp3-trigger-downloads/
#VOLM 0 - sets the output volume of ther board to maximum.
If you create the same .ini file on each board, the output shall be the same on all your boards.
If you use a small speaker on each board (12 speakers in this case), they shall all play at the same volume.
Remember that sounds are the most difficult things to manage in a museum or science centre.
Concerning the delay, try to open the sound file with Audacity (it’s free and very powerfull tool). Don’t forget to download ant install the add-on LAME for Audacity.
Open your file and then export it as .mp3. There shouldn’t be any “blank” time at the beginning.
I hope that this will help you.
Best regards,
Yvan.