Click the waveform to move the selection there and hear it.
Sketches that build up — particles, trails, flow fields — start almost black. Warm-up draws these frames first and throws them away, so the video opens on a finished image instead of fading in. Use RENDER_T rather than frameCount for your timing when warm-up is on.
Your sketch can read RENDER_W, RENDER_H, RENDER_FPS and RENDER_FRAMES. Use RENDER_T — a number running 0 to 1 across the clip — as your time variable and it loops seamlessly. RENDER_FRAME is the same thing as a frame number. Time inside the sketch is virtual, so a slow frame never drops. With a track loaded in spectrum mode you also get RENDER_SPECTRUM (48 bands, 0–1), RENDER_LEVEL and RENDER_AUDIO — analysed offline, so every value is exactly the spectrum at that frame. Browsers without an AAC encoder (Firefox, Chromium on Linux) can still add sound from a .m4a or .aac file — those frames are copied in untouched.