Today I have taken delivery of the lovely little enclosure that is going to house this project, along with the 10k stereo pots required to control outputs of the sound modules in the DIY mixer part of my plan. Once the stereo jack sockets arrive I'll be building this unit, testing it, then giving it to our drummer for feedback in order to fine tune the code.
The Arduino will be reading the pad inputs and the volume/sound bank encoder, as well as outputting to the LCD screen and outputting serial to the four sound modules.
The sound modules will then play the requested sound files via a Left and Right output. In order to mix these four left and four right outputs into one main left and right out voltage dividers must be used to avoid adding too much noise to the output. My voltage dividers will be made up of 1k resistors and 10k potentiometers, which will allow variable control of the pad outputs. In order to keep the interface clean I am using stereo pots instead of single pots to a)reduce the number of knobs on the interface and b) maintain even control of both L&R outputs of each sound module.
Here's a shot of the enclosure and all the parts, except for the panelmount usb cable and the audio jacks: