When you build qt-mobility, the debug version of declarative plugin is not build with d suffix. This means the dll for debug and release has the same name, and building the release DLL overwrites the debug DLL.

running QML with Qt.multimedia;

import Qt 4.7
import Qt.multimedia 1.1
Rectangle {
    width: 200
    height: 200
  
    Video {
   id: video
   anchors.fill: parent
   source: Qt.resolvedUrl("C:/TestFolder//test.wmv")
   Component.onCompleted : video.play();
   }
}

results in

QMediaPluginLoader: Failed to load plugin: "C:/QtMobility/plugins/mediaservice/dsengine.dll" "The plugin 'C:/QtMobility/plugins/mediaservice/dsengine.dll' uses incompatible Qt library. Expected build key "Windows msvc debug full-config", got "Windows msvc release full-config""
QMediaPluginLoader: Failed to load plugin: "C:/QtMobility/plugins/mediaservice/qtmedia_audioengine.dll" "The plugin 'C:/QtMobility/plugins/mediaservice/qtmedia_audioengine.dll' uses incompatible Qt library. Expected build key "Windows msvc debug full-config", got "Windows msvc release full-config""
QMediaPluginLoader: Failed to load plugin: "C:/QtMobility/plugins/mediaservice/qtmedia_wmp.dll" "The plugin 'C:/QtMobility/plugins/mediaservice/qtmedia_wmp.dll' uses incompatible Qt library. Expected build key "Windows msvc debug full-config", got "Windows msvc release full-config""