The Morserino cannot talk to Mumble directly, you need to use a computer in between. Mumble is a VOIP protocol with very low latency, which makes it suitable to use for CW. Morse code is transmitted as audio in this case. As the Morserino has a separate audio I/O jack, you can use that to feed the Morserinos audio into the computer, and also to get the computers audio into the Morserino, where it will be „re-shaped“ and both played on the loudspeaker, and also decoded and shown on the display.
The attached document describes how to build a cable to interface the 4 pole Morserino jack into two separate audio jacks for a PC or sound card with separate microphone-in and audio-out jacks.