docs: loadSound should not use preload

[ksen0]

[NisargJasani0602]

The loadSound() reference doc in SoundFile.js has been updated on main in commit 86c3bcf ("SoundFile refs now use async await for loadSound"). It no longer mentions preload() or a callback — it now documents calling loadSound() with await from an async setup().

[NisargJasani0602]

While checking for other stale references, I noticed four other files whose @example docs still call loadSound() inside function preload() {}, which is now inconsistent with the updated loadSound() behavior:

src/effects/Panner.js (two examples)
src/effects/Panner3D.js
src/analysis/Amplitude.js
src/effects/Gain.js
I'll open a PR updating these to use the async function setup() + await loadSound(...) pattern, consistent with SoundFile.js, Reverb.js, and Delay.js.