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The only supported sound output is currently JACK. You should make
sure jackd is running (jackd -r -d oss), then enable it in MuseScore
preferences:

Edit -> Preferences -> I/O -> [X] Use internal synthesizer ->
                                             -> [X] JACK Audio Server

Restart MuseScore, and the sound should work. You may still need to
select different JACK ports for left/right channel in preferences.

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Also, CJK users may want to install some of

chinese/qt4-codecs-cn chinese/qt4-codecs-tw
japanese/qt4-codecs-jp korean/qt4-codecs-kr

ports for legacy-encoding text to be displayed correctly.

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