Presumably you changed speakers after starting MuseScore. In that case, just toggle the MIDI button on the toolbar off then on again to force MuseScore to reinitialize its list of devices and switch to the new one (same as going to Edit / Preferences and hitting the "Restart Audio and MIDI Devices" button).
If that doesn't fix it, please provide more information - what version of MuseScore, what OS, and precise steps to reproduce the problem.
Comments
Presumably you changed speakers after starting MuseScore. In that case, just toggle the MIDI button on the toolbar off then on again to force MuseScore to reinitialize its list of devices and switch to the new one (same as going to Edit / Preferences and hitting the "Restart Audio and MIDI Devices" button).
If that doesn't fix it, please provide more information - what version of MuseScore, what OS, and precise steps to reproduce the problem.