If you don't plan on releasing this, You could extract the treasure chest, modify it, then repackage it.
Then set it to read-only so blockland doesn't overwrite it when the launcher starts. :P
To add a new sound, you'll want code like this:
datablock AudioProfile(partyhorn_Sound)
{
filename = "./partyhorn.wav";
description = AudioClosest3d;
preload = false;
};
And then you could change line35 to:
%par1 = partyhorn_Sound.getId();
A better option would be to package up brickTreasureChestData::onPlant(%data, %obj)
but I don't want to explain that