This might be a bit off topic, but you may be interested in an alternate audio library I've been writing for Teensy. It's still at a fairly early stage of development. A major feature (which does work well) is playback in the background. In fact, this new library lets you create many audio objects, interconnected in arbitrary ways, with automatic audio data flow between them, all updated in the background. For example, you can create 3 wav file players, connect them to the inputs of a 4 channel mixer, and then connect the mixer to the DAC output, or an audio shield output, or both.
Unfortunately, this new library requires different hardware. While the audio processing objects are relatively pure software, all the input & output objects are coded for the DMA hardware on Freescale's chip. The library also makes heavy use of the Cortex-M4 special DSP accelerating instructions, which dramatically speed up some objects like the FFT, BiQuad filters, and waveform synthesis.