Audio: what do I need to do to get SPDIF data to a speaker?
You need an audio DAC, one way or another. Either a device that can directly receive S/PDIF and output analog audio, in which case it might be a more complex audio codec chip, or simply an audio DAC which only has an I2S audio input, and then you only need an S/PDIF receiver which can output the audio data over I2S bus.
S/PDIF is not a compressed data format. It is simply an interface to stream digital audio data, and it can be used to transmit either linear PCM (LPCM) sample data or encoded bitstream data. When data is not LPCM but encoded bitstream, it must first be decoded with a DSP to end up with LPCM data that can be fed to DAC.