I know Apple likes us to think everything works by magic, but...
To use Airplay one needs Airport Express. To backup wirelessly one needs Time Capsule. There is, in particular, no way to Airplay music through Time Capsule, as one can do with Airport Express. I hope there is a better reason for this than selling these two devices. (Note also that the 4th-gen AppleTV + Time Capsule are also not a satisfactory solution for those whose HiFi lacks HDMI.)
My question is: if one has both Airport Express and Time Capsule, what are the terms of their co-existence? Does one, for instance, get to choose which of the two is the wifi router? If one chooses Time Capsule, would Airport Express be using the same band (2.4GHz? 5GHz?) for transferring the music? Is the quality of Airplay affected if one is using the bandwidth to capacity?