MuseScore.com uses the default MuseScore SoundFont “TimGM6mb” to playback your score online. So in case you made your score with a different SoundFont, the online playback will sound different. Join the Improving MuseScore.com group and add add your voice to this discussion if you like to see this changed.
Go to the score page, click on the 'Edit this score' link. At the bottom of the editing form you will find the delete button.