I believe that making the audio input available as a layer in the standard zynthian interface( if you are using an audio injector card) would allow an simple way of providing an external input (cd, player, audio input etc) with a volume control to a zynthian.
It would simply consist of an extra layer engine… audio In with a single volume control.
At present we have no way of providing such a facility without using the MOD which requires a separate web interface.
Until we have a way of playing ogg/mp3/wav files then this provides an easy way of providing the ability to play along with a sperate audio source without requiring an extenal mixer.
A personal use case is using a zynthian as a guitar amp so I can monitor using the headphone jack and play bass pedals using the zynthian via MIDI. This way I get control of all my signal outputs before sending it to the on stage mix. and it saves stage clutter.