Sorry, for various reasons Spine is limited to reading WAVs with 16 bits per sample. Internally Spine is using 16 bits and video export is also 16 bits, so to support 24 bits it would need to down sample. We could do that in the future, but for now we only accept 16 bit WAVs.
If you are using the Spine Runtimes, note you can use 16 bit WAVs in Spine and completely different audio files (24 bit WAV, or even a different format) at runtime, since it is up to you to respond to the audio event to trigger playback.
While likely you are already familiar with audio tools, FWIW I have found sox to be a great tool for converting audio. Eg, to convert 24 bit WAV to 16 bit:
sox the24bit.wav -b16 the16bit.wav