I've been working some with the audioengine we use in mC2. The engine was originally a rip of the tuniac eninge, but has been modified by bjorn to fit our needs in mC2 (really great work bjorn). The last month I've been implementing the filestreams in the decoders and started to look at how we should implement DSP and mixing plugins.
I found a library that looks very interresting called SoundTouch that I can see some great features in.
First of all we could use it for "Track analyzer" plugin. A "Track analyzer" plugin is a plugin that make some kind of anayze of the sound and saves that info to the tracks metadata. SoundTouch could be used for analyzing BPM in all your tracks.
I also has been thinking of some more advanced plugins like DSP/Mixing plugins like beatmix plugins with timeshifting beats for a perfect mixing between tracks. SoundTouch is perfect for that kind of plugins.