midiFILTr-VT is an Audio Unit v3 MIDI Effect and CoreMIDI capable app that allows you to play the individual drum parts on your Volca Beats drum machine with varying volumes using MIDI velocity.
The Volca Beats has a MIDI spec that does not listen to the velocity of incoming MIDI notes. midiFILTr-VT allows you to play MIDI notes for each drum part with a velocity that is mapped to the part volume by adding a MIDI CC message along with each note that controls the note's velocity. The harder you tap the drum pad or keyboard, the louder the note. You can also exclude certain parts from this so that only the kick and snare vary in volume while the other parts remain constant in the mix.
Additionally the Audio Unit version allows you to control the MIDI CCs of the Volca Beats using AU parameters for easier MIDI mapping and automation of the drum machine.
This app requires:
A hardware Volca Beats
A hardware MIDI interface connected between your iOS device and the Volca Beats
An Audio Unit host that supports AUv3 MIDI Effects such as AudioBus or AUM is recommended for ease of use, though the app can also be configured with regular CoreMIDI.