This is not a bug; the default is set so that loud passages do not clip. But for future versions, there will be a compressor effect that can help normalize volume levels (at the expense of musicality, as is always the case when compressing dynamic range).
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Which version of MuseScore do you use? Have you checked the mixer panel (View > Mixer F10) and increased the volume?
It's better to use the forum for support questions. Once we consider it a bug, we can file it in the issue tracker.
This is not a bug; the default is set so that loud passages do not clip. But for future versions, there will be a compressor effect that can help normalize volume levels (at the expense of musicality, as is always the case when compressing dynamic range).
Thanks for the great answer Marc!