There is quite a lot of software out there to do multichannel I/O for Linux (Raspberry Pi). For example LADSPA http://www.zthmusic.com/ladspa-plugins-for-mixing/. Things like delay, EQ, compression, reverb, etc.
There are also similar apps for jack audio.
Here is a nice list of many :
Here is a great HOWTO by Richard Taylor:
You can also use vst plugins with ALSA :
There are many DAW for audio editing, both free and commercial.