How to drive a cd4051 analog mux with buttons as channel selector?
Use a priority encoder like a 74xx148



Or if you prefer CMOS a CD4532



Only one switch should be active at a time so this worked best with ganged switches, either push buttons like the one below or a multi-position rotary one. 



But then again, one has to wonder, if you have all those switches, why do you need the CD4051?