Yeah, I found a tutorial on how to do one in an Altoids can, but I wasn't really looking for a new project.

I guess my Google-fu is stronger today, because I just discovered the PAC ASAI21 that looks like it might do what I want (diagram). It goes the switching route rather than the mixing route, but it does so automatically, which might work out pretty well.

I'll keep looking for something that mixes or cross-fades or something, but if I can't find that, I can probably do just fine with one of those.