Is there any official pronunciation guideline for how to pronounce "slaad"?
Dragon #93 (1985) includes the article "Ay pronunseeAYshun gyd", written by Frank Mentzer, which has guidance on how to pronounce the names of a vast array of monsters from 1e AD&D, including Slaad (the article has notable errors regarding how to pronounce some genuine English words, but can probably taken to be authoritative on its made up ones). The Forgotten Realms wiki article on Slaad includes that guidance in IPA notation, there apparently being two accepted pronunciations:
pronounced: /slæd/ slæd or: /slɑːd/ slad
The 2e resource Monstrous Compendium - Outer Planes Appendix also includes pronunciation guides for its monsters in its index, giving:
If D&D Beyond counts as official, lots of the monster entries include VO of someone pronouncing their name - the core Monster Manual entries are voiced by Matt Mercer and Marisha Ray of Critical Role. The sound bites can be played by clicking the speaker icon next to the monster's name on their page and seem (to my ear at least) to agree with the suggested pronunciation of the prior resources.