Is it possible to create a way to have a trigger in Roll20 cause a one shot sound to play? Example:player casts fireball in roll20, fireball spund plays. (Without me having to trigger it each time?)
Sadly not no.Syrinscape is designed to allow this to happen, that’s how extensions have been created that allow it on lots of other VTT’s such as Fantasy Grounds. But Roll20 will not allow the triggers, as they have their own media player.
If it’s something you would like to see then make sure to tell them on their forums 
Thanks for the response. I searched a ton, so I figured that was the answer.
What Roll20 will have to be able to do, is to be able to create an object that you can attach a LINK to…
This is the Fireball link in Syrinscape:
https://www.syrinscape.com/online/frontend-api/elements/20/play/?auth_token=[Insert YOUR user token here (can be copied from your /cp/ page)]
(ideally Roll20 will catch the browser page this opens so the system doesn’t display that)
OR
syrinscape-fantasy://www.syrinscape.com/online/frontend-api/elements/2459/play/?auth_token=[Insert YOUR user token here (can be copied from your /cp/ page)]
(this will NOT open a browser window, but will add a 0.5 second delay to the call)
What about finding someone in the Roll20 community who might REALLY find out if this is possible or not?