Site playback
Musescore (the site itself) doesn't seem to be able to accurately play dynamics, but the program does. The uploaded version of my score (https://musescore.com/arpice/hungarian-rhapsody-no-6-orchestral) sounds extremely weird in some parts because some dynamic markings are either ignored or extremely exaggerated. You can barely hear the first clarinet in the lassan section.
Comments
See: https://musescore.org/en/node/277874
You'll have to ask the question at the other site, e.g.: https://musescore.com/groups/improving-musescore-com
Are you by chance using a non-default soundfont? If so you need to be sure to check the "Upload score audio" button when saving online.