In order to reproduce:
 Create new text document.
 Open Gallery with menu: View > Gallery
 Select theme 'Sounds'.
 Double-click a displayed sound. The Media Player is opened and plays the sound.
 Dock the Media Player right or left.
 Try to close the Media Player. I did not find an obvious possibility to close the Media Player. Even if you close and start LibreOffice again the Media Player will still be displayed. Expected: Obvious possibility to close the Media Player.
 I found these possibilities to close the Media Player:
(a) With a left click into the bottom right corner below the list box you can pull up an undocked Media Player. The undocked player can be closed.
(b) In the Customize dialogue you can add the command Media Player to a menu, a toolbar or shortcut key. Afterwards the player can be closed with the added command.
(c1) Open toolbar Media Playback.
(c2) Click 'Media Player' in toolbar. The Media Player is closed. But:
(c3) Double-click a displayed sound in the Gallery again. Now the media player is displayed incompletely (Speaker is not displayed) and the sound is not played.
Bug also exists in Impress and Draw.
In Calc it's even not possible to play a sound at step 4.
Bug already exists in version 3.3.0. Hence inherited from OOo.
Tested with version 6.0.5 (64 bit, Win 10).
Version is earliest version that the bug is displayed.
Confirmed. Enhancement request.
Build ID: 9b0d9b32d5dcda91d2f1a96dc04c645c450872bf
CPU threads: 4; OS: Windows 6.1; UI render: default;
Locale: en-US (en_US); Calc: CL