Is there a connector I can solder onto that board, to work with the plug on those speakers? The description refers to an SPK cable, which I can’t find much on, or do I just need a JST connector?
That Audio FX board comes with two terminal blocks that can be soldered into the speaker output pins so the easiest way to connect the speakers you linked would be to cut off the JST plug and just clamp them into the terminal blocks.
If you did want want to use a JST connector those speakers come with a JST PH (2.0 mm pitch), which I don’t think will fit the hole spacing on the Audio FX module.
FYI, Adafruit have put together a killer tutorial for this module, so if you haven’t come across that yet, it’s definitely worth checking it out.