Sometimes you need to take several mono or stereo wav files and collate them together into a single multi-channel file. This is particularly pertinent for ambisonics and for B-format audio in general. I'm writing this because I couldn't find a decent set of instructions online and I had to figure it out myself.
Yesterday I had four mono files that I had captured with an ambisonic mic and a field recorder. The field recorder spits out a mono wav file for each of its inputs. I wanted a single, four-channel file that I could process and spatialise in Reaper using ambisonic decoding plugins.
I took the four files and dragged them into Reaper on separate tracks. When you drag in a bunch of files it asks whether you want them on separate tracks or the same one. Hit the separate tracks button.
Then I clicked on the I/O button on the master bus and set it to have 4 track channels instead of two.
After that I made a new track and did the same thing; setting it to four channels in the I/O menu. Then I made a send from each of the four mono channels (the ones with my audio in them) to the new 4-channel track. In the I/O window of each of the audio tracks, turn off the master/parent send so the audio doesn't mix together in the master bus. At the bottom of the send box in the I/O window, you can set how many and which channels (from the track you're sending from) go to which channels in the recieving track.
I set my first audio channel to have 1 mono send, sent to channel 1 on the recieving track. My second track I set to have 1 mono send, sent to channel 2 on the receiving track; and so on. When I finished all of that I went to File > Render and rendered out my 4-channel wav file. You have to specify 4 channels in the Render window too.
I hope this quick guide was useful and that it saves someone much time and brain-hurt. Also: Future Morgan, if you forget how to do this, you're welcome.