Zynthian supports class compliant USB audio soundcards so a device that acts as a class compliant audio interface can be used as Zynthian’s only audio interface.
Zynthian also supports gadget mode for MIDI only. If you can provide power, e.g. via a splitter cable then Zynthian’s USB C power connector can be connected to a host device and Zynthian is seen as a class compliant MIDI device. (This is little used for to the complexity of supplying sufficient power.) Audio is, in theory possible but not currently implemented (upstream).