You need to convert from a IMA ADPCM formatted .wav to Playdate’s own .pda format. The easiest way is to install Panic’s (who make the Playdate) Nova IDE and add the Playdate extension:
Once the extension is installed create a new project, then add the extension: Project > Project Settings > Tasks and add the Playdate task. If you then add a .wav tot he project and build (Project > Build) the Playdate compiler will convert from .wav to .pda - you can then drop the .pda in the Granular directory on the Playdate (put the Playdate in disk mode via Settings, then it’ll appear as a USB disk on your computer).
I’ll try and find time to build a drag-and-drop desktop tool to make this easier, but time is scarce at the moment.