I'm using the sands clip+, rly like the recordfunction as well. Is there any plugin out there to maybe mix some recorded tracks together?
At the most basic level, a WAV file is nothing but a stream of numbers indicating how far out the speaker cone was at a certain instant during the recording (see also: pulse-code modulation).
Doing simple things, like mixing or concatenating two tracks, is completely possible, even on Rockbox (it might even be possible from the text editor).
It's just a matter of writing the code. However, adding more flexibility and functionality to such a plugin could be an issue.
Also, once you get into more complex tasks, the processing power of the devices will likely become a roadblock.
If you want to see it happen, the best person to go to is... YOU!