It's not easily to find out what the sound files in {code}MediaPortal\Source\UI\UiComponents\SkinBase\Skin\default\sounds{code} are used for.

It's suggested to rename them as follows and adjust the relevant skin files:
button12.wav > navigate.wav
button7.wav > click.wav

For browsing the folders history see:
http://issues.team-mediaportal.com/fisheye/changelog/~br=dev/MediaPortal-2/MediaPortal/Source/UI/UiComponents/SkinBase/Skin/default/sounds