Firstly, please be aware that the line and headphone outputs have separate volume controls, see Osmose's hardware overview.
If you still have no sound, please check if your local control option was disabled:
Press the "m" mode button to go to mode selection screen -> global settings ⚙️ -> midi i/o tab -> haken local control. Make sure "haken local control" is switched on. Otherwise, the keybed is disconnected from the sound engine.
ℹ️ If you find the local control option was turned off but cannot remember turning it off yourself, it might have been caused by an internal function preventing a MIDI feedback loop. It turns local control off when detecting the onboard DSP's ping signal on the Osmose's MIDI input. For details, see the midi i/o tab in global settings.
If Osmose still doesn't emit any sound, even with haken local control set to "on", a factory preset loaded, and the volume turned up, please try reflashing the DSP firmware via OsmoseUpdater's Advanced Mode. Be aware that this step initializes your user presets, so back those up before the process when needed. Flashing the DSP firmware and backing up the user data is explained in the Osmose firmware update instructions -> Advanced Mode -> Update Haken firmware.