MSP  [protocol]
The MUD Sound Protocol — the server names a sound file and the client plays it.
Old, simple, and easy to confuse with two other things.
https://www.zuggsoft.com/zmud/msp.htm

Measured adoption
  0 of 886 listed games were observed offering it (0.0%)
  /games?protocol=MSP

  The games not counted here are not games without the protocol. A game is
  counted when we observed its server offering the option in a handshake; the
  rest are servers that did not offer it to us and servers whose handshake we
  have not read, and we cannot tell you which.

  By codebase, of the games we identified
    AresMUSH             0 of 18 offered it
    CircleMUD            0 of 47 offered it
    CobraMUSH            0 of 2 offered it
    CoffeeMUD            0 of 32 offered it
    DikuMUD              0 of 35 offered it
    Evennia              0 of 11 offered it
    FluffOS              0 of 28 offered it
    MUCK                 0 of 24 offered it
    PennMUSH             0 of 34 offered it
    RhostMUSH            0 of 40 offered it
    ROM                  0 of 55 offered it
    SMAUG                0 of 20 offered it
    tbaMUD               0 of 5 offered it
    TinyMUSH             0 of 9 offered it

MSP lets a server ask a client to play a sound: a bracketed directive naming a
file, a volume, a repeat count and a URL to fetch it from if the client does not
have it. It negotiates on telnet option 90, and it can also be sent in-band in
the text stream by servers that never negotiate anything.

It is genuinely old and genuinely still used — ambient sound in a text game is a
bigger effect than it sounds like, and for players using a client's audio cues
rather than its display it is more than decoration.

THREE THINGS IT IS NOT

The client tables in this section had to be careful here, and it is worth
writing down why:

- MCMP — the Mud Client Media Protocol — is a different protocol doing a similar
  job. At least one client implements MCMP and not MSP, and reading one as the
  other would put a claim in a table that nobody made.
- A client's own "play a sound" scripting call is not MSP. It plays a local file
  when a script says so; MSP is a server telling a client what to play.
- Bundled-plugin support is worth stating as such. One client's MSP support
  ships as a plugin that explicitly does no telnet negotiation, which works on
  servers that send MSP in band and not on servers that expect to negotiate it.

WHAT WE MEASURE

Servers offering telnet option 90. Because MSP is frequently sent in band
without negotiation, this figure understates deployment by an amount we cannot
estimate — which is a limitation of what a handshake can see, and not a finding
about the protocol.

See also
  MXP — /reference/protocols/mxp
  VIP Mud — /reference/clients/vipmud