[ubuntu] Libreoffice Impress - control sound volume?

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http://ubuntuforums.org – I recently got some Power Point slide shows (pps files) that combine picture slides with background music. They open full-screen and play; I can advance the slides and hear the music. I know these files weren't designed for Libreoffice Impress, but I haven't figured out how to reduce the sound volume. No matter where my volume control is set, the pps plays at maximum volume. I can reduce system volume or even mute system volume with no effect. I haven't been able to access application volume in the upper taskbar because the pps opens at full-screen and hides the upper taskbar. Any recomm (Hardware)