ANSI music
Escape sequences that played a tune on the PC speaker when you loaded a BBS screen.
An extension of the ANSI codes: the sequence ESC[M followed by a note string in the same style as GW-BASIC's PLAY. Terminal programs such as Telix and Qmodem parsed it and bleeped out the notes, often paired with an ANSI art screen so a login could be both seen and heard.
Never an official standard — support lived in the terminal, not the server — and just as often switched off by people sick of shrill welcome jingles. The trick died with the BBS era, roughly when the PC speaker itself gave way to real sound cards.