Well, as far as I know, QMultimedia and QMultimediaWidgets are not present in the Qt 6.1 release. If you look att the following list, there are all the removed modules. (https://doc-snapshots.qt.io/qt6-dev/whatsnew60.html#removed-modules-in-qt-6-0)
Also, if you look at the Qt 6.2 available modules, they don't appear neither: https://doc-snapshots.qt.io/qt6-dev/qtmodules.html
Qt claims that those modules may be included in a future release (i hope this happens), but for the moment you'll need to stick with Qt5.
(Sorry for my english)
EDIT:
Starting from Qt6.2, the QMultimedia module is available again (but QMultimediaWidgets still not present)