After writing my own sampler and mixer with XAudio2, I can tell you that there was little information easily found on the internet at all. I have previously noted that gamdev does not have a specific forum devoted to audio programming.
I assume that your prior knowledge on DSP is not code related, but on how these work on waveforms. If that's the case, I would look into how to manipulate waveforms and attempt to recreate the tools of trade as used in the studio, amplitude effects such as compressors/limiters or expanders, delay effects like flanging and reverb, and filter effects like phasers, EQ, and wahs. This would tap into your knowledge of DSP and give you stuff to work on with your hobby coding.