The little 6 is a single-character substitute for "\^", which is a common control code. Zep didn't necessarily make all the shortcuts like that very obvious. I actually included characters 0-15 in the tool, they're just in the code as comments and you can copy and paste them, since they're not printable in a running program.
Ok I get the doc is split in two on fandom https://pico-8.fandom.com/wiki/P8SCII make me realise that we can save an extra char when play sound :
?"⁷e6" instead of ?"\ae6"
Also yes indeed that made them not printable :) (which it's sad because the glyph looks cool) I'll download the cart then.