You cannot add the code directly into the Bitsy interface as far as I am aware. You will need to add it to the html file. I usually just open my games up in notepad and go from there once I hit the adding sound stage.
Explanation using my game The Opposite of Brontide as an example below, let me know if it's not clear.
First, find the part of the hack that says the entries below are examples and put in your sound file names and what you'll refer to them as:
'wind': { src: './lightwind2.wav', loop: true },
'lightning': { src: './lightningcrackle.wav', volume: 0.5 },
'thunder': { src: './thunder1.wav', volume: 1 },
Next, find musicByRoom and add tracks as necessary:
musicByRoom: {
0: 'wind',
1: 'wind',
2: 'wind',
3: 'wind',
Then for specific sfx, find the parts of the dialogue that you want to have them in and call the various sounds there
DLG 1
"""
{
- {item "2"} == 1 ?
(exitNow "3,+0,+0")Spreading outward from your ribcage until your ears
- {item "2"} == 2 ?
(soundeffectNow "thunder")finally catch up