There are certain invocations a Warlock can get such as Bewitching Whispers which states:
[...] You can cast compulsion once using a warlock spell slot. You can’t do so again until you finish a long rest.
This means that an ordinary Sorcerer/Warlock multiclass could not use spell slots from their Spellcasting feature (Sorcerer spell slots) with Bewitching Whispers. However, the Sorcerer also gets the Flexible Casting feature which states:
[...] You can transform unexpended sorcery points into one spell slot as a bonus action on your turn [...]
I am aware that the multiclassing rules to allow features such as Flexible Casting and Metamagic to apply to Warlock spells as they state:
Multiclassing allows you to gain levels in multiple classes. Doing so lets you mix the abilities of those classes to realize a character concept that might not be reflected in one of the standard class options.
If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
However it is unclear to me whether spell slots created through Flexible Casting count as Warlock spell slots for the purpose of things such as Bewitching Whispers. Can Flexible Casting spell slots be used with this invocation?