According to 9to5Mac, they have discovered an FCC filing for an accessory made by Belkin that will allow users to retrofit AirPlay 2 connectivity to speakers that don’t have the feature. Such an accessory isn’t exactly new as Apple had in the past created the AirPort Express, which has since been discontinued.
This adapter made by Belkin has been referred to in the filing as the Belkin Soundform Connect which is powered by a USB-C cable and will support audio output through a 3.5mm port as well as Optical Digital Audio. Basically once a speaker is connected to the accessory, Apple devices like the iPhone or iPad will recognize it as an AirPlay 2 compatible speaker and will be able to stream audio through it wirelessly.
That being said, it should be noted that there are already accessories that give speakers Bluetooth connectivity, but in case AirPlay/AirPlay 2 is your preferred wireless connection, then this could be an accessory to keep an eye out for.