Three bills, one video, that's it. The $1 bills had a nice stiff, crackly sound that I thought would work well in a video, paired with another, less crackly bill. Just a bit of whispering in the beginning.
To listen while using other apps, switch the player to Picture-in-Picture (PiP) during playback — it keeps playing in a small floating window (the screen stays on).
To listen with the screen fully off, in-browser playback stops by YouTube's design. Open the video in the YouTube app to keep listening where background playback is supported (e.g. with YouTube Premium).
Open in YouTube app