See the documentation ( http://tmrh20.github.io/RF24Audio/classRF24Audio.html ) and examples for info about volume control. For external volume control, just connect button(s) as per the documentation.
The library works best currently at about 16-24khz sample rate due to how it works. The TX_0 development sketch (GitHub - TMRh20/Sketches: Arduino Audio Streaming Realtime, NTSC Video) supports faster rates, but is more complicated to use.