There are two ways to output audio from the computer running Museik.
Option one is to plug in to the 3.5mm headphone jack. This will give you a decent stereo signal that can be routed to external speakers, a sound board, or other external audio equipment.
Option two is to connect a USB audio interface using a USB cable. This will give you a superior audio signal with minimal extra hassle.
In either case, be sure to go into the SETTINGS window and 1) verify that audio is set to ON (Muséik will intentionally turn audio OFF when a new external audio device is connected), and 2) select the correct audio output device.