The docs for the various audio related QML objects all explain that volume is linear and QAudio::convertVolume should be used:
https://doc.qt.io/qt-6/qml-qtmultimedia-audiooutput.html#volume-prop
https://doc.qt.io/qt-6/qml-qtmultimedia-audioinput.html#volume-prop

However QAudio::convertVolume and the associated enums are not available in QML
https://doc.qt.io/qt-6/qaudio.html#convertVolume

In QtMultimedia 5.15 convertVolume and the enums were exposed to QML in a global QtMultimedia namespace
https://doc.qt.io/qt-5/qml-qtmultimedia-qtmultimedia.html#convertVolume-method

Something similar needs to be added to Qt6 so that QML can expose volume controls that scale logarithmically.