Select the right output device to get the best sound
When you have installed a new synthesizer, it is essential to ensure that Karaboss will use your new output device (the synthesizer you have installed). Otherwise it may continue to use the default synthesiser supplied by Microsoft (Microsoft GS Wavetable Synth).
Why?
In the sound production chain, Karaboss plays the role of sequencer. It needs a synthesiser to produce sound.
The diagram below shows that to produce sound (PLAYBACK) you need to connect the sound output of the Karaboss sequencer (MIDI OUT) to your synthesiser.

Source https://www.synthfood.fr/choisir-votre-sequenceur-et-synchroniser-vos-synthetiseurs/
Display the output device settings window
From the explorer, click on the MIDI > Output device menu to display the output device selection window.

Select an output device
Choose your output device from the list of available output devices.

There will be at least the one provided by Windows (Microsoft GS Wavetable Synth) and, in principle, the device(s) you have added to your system.
Once you have selected your output device, tick the ‘Save output parameter’ box so that you don’t have to do this again.
